The chrome directory and userChrome.css file does not exist by default. It will need to be created.

: userChrome.css Tab Center Redux zDark Theme (Modern).css

[Firefox 57.0b11->65.0b12 Developer Edition]
Tab Center Redux 0.5.1 zDark Modern Theme

/* Place the Code below inside the chrome folder in a file named userChrome.css */ 
[%APPDATA%\Mozilla\Firefox\Profiles\[YourProfileID].dev-edition-default\chrome\userChrome.css */

@namespace http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul;// userChrome.css JZersche //
#navigator-toolbox{margin-top:1px}#TabsToolbar{visibility:collapse!important}#main-window{--toolbar-bgcolor:#282C30!important}#urlbar{background:#1f2226;border:1px solid #33383c}#pageAction-urlbar-reload-in-urlbar_exe-boss{filter:invert(100%)}#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"]{overflow:hidden!important;min-width:32px!important;max-width:32px!important;position:relative!important;top:89px!important;transition:all .2s ease!important;border-right:2px solid #0088eeff!important;z-index:2!important}#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"] #sidebar{position:relative;height:100.25%!important;top:-1px}#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"]:not([hidden="true"]){margin-left:0!important;background-color:#06a!important;margin-top:0!important;border-top:0 solid #ff363b!important;display:inline-block;border-top-right-radius:0!important}#sidebar-splitter #appcontent{margin-left:0!important;background-color:#0ff!important}#browser-panel{background-color:#222!important}#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"]:hover,#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"] #sidebar{min-width:210px!important;max-width:210px!important}#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"] #sidebar-header,#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"] ~ #sidebar-splitter{display:none}#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"] #sidebar-header{background-color:LIME!important;border-bottom:1px solid red!important}#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"] #sidebar{height:calc(70vh - 16px);border-top:1px solid #272b2f!important;margin-top:0!important}#main-window[inFullscreen] #appcontent{margin-left:0!important}#main-window[privatebrowsingmode="temporary"] #TabsToolbar,#main-window[inFullscreen] #TabsToolbar{background-color:#fff}#main-window[privatebrowsingmode="temporary"] #tabbrowser-tabs,#main-window[inFullscreen] #tabbrowser-tabs{visibility:visible!important}#main-window[inFullscreen][inDOMFullscreen] #tabbrowser-tabs{visibility:collapse!important}#main-window[inFullscreen] #sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"]{display:none!important}#navigator-toolbox > toolbar:not(#TabsToolbar):not(#toolbar-menubar),.browserContainer > findbar,#browser-bottombox,#toolbar-menubar{background-color:#282c30!important;border:0 solid transparent!important}#TabsToolbar-customization-target{background:#282c30!important}#nav-bar-customization-target{margin:0 2px}#window-controls{filter:invert(75%)}#nav-bar toolbarpaletteitem[id^="wrapper-customizableui-special-spring"],#nav-bar toolbarspring{-moz-box-flex:1!important}.findbar-button:hover{color:#ddd!important}.findbar-button[checked="true"]:not(:hover){color:#fff!important}:root[uidensity="compact"] #nav-bar[brighttext] > #PanelUI-button{border-image-source:linear-gradient(transparent 4px,rgba(100%,100%,100%,.2) 44x,rgba(100%,100%,100%,.2) calc(100% - 4px),transparent calc(100% - 4px))}:root[uidensity="compact"] #PanelUI-button{margin-inline-start:3px;border-inline-start:1px solid;border-image:linear-gradient(transparent 4px,rgba(0,0,0,.1) 4px,rgba(0,0,0,.1) calc(100% - 4px),transparent calc(100% - 4px));border-image-source:linear-gradient(transparent 4px,rgba(0,0,0,0.1) 4px,rgba(0,0,0,0.1) calc(100% - 4px),transparent calc(100% - 4px));border-image-slice:100%;border-image-slice:1}#navigator-toolbox::after{content:"";background:#303030!important;display:-moz-box;border-bottom:1px solid #303030}.PanelUI-subView .toolbaritem-combined-buttons > .subviewbutton:not(.subviewbutton-iconic){padding:0 10px!important;background-color:#09f!important;border:0 solid #222!important;border-radius:6px!important}menupopup:not([id="BMB_bookmarksPopup"]),#BMB_bookmarksPopup menupopup[placespopup=true] > hbox{background-color:#064e9d!important}:root{--uc-dark-bkgnd-color:#064e9d!important}.panel-subview-body{overflow-y:auto;overflow-x:hidden;-moz-box-flex:1;padding:6px 0;background:#064e9d;background-image:url(https://i.imgur.com/4tlUOTz.png)!important;filter:grayscale(100%)!important}:root{--uc-border-color:#06a!important}#navigator-toolbox::after{content:"";display:-moz-box;border-bottom:0 solid var(--chrome-content-separator-color)!important}.panel-subviews,#mainPopupSet menupopup,#bookmarks-menu-button menu menupopup > hbox,.search-panel-header.search-panel-current-engine,.search-panel-current-input,panelview,.PanelUI-subView{border-top:0 solid #064e9d!important;background-color:var(--uc-dark-bkgnd-color)!important}panelview{color:#fff!important}.subviewbutton:hover,#PanelUI-contents toolbarbutton:hover{background-color:#06a!important}menupopup:not([id="BMB_bookmarksPopup"]),#BMB_bookmarksPopup menupopup[placespopup=true] > hbox{-moz-appearance:none!important;border-radius:0!important;font-weight:400!important;border:0 dotted #0066aacc!important;border-left:0!important;border-top:0 solid #09f!important;border-radius:0!important;box-shadow:0 0 0 rgba(0,0,0,0) inset,0 0 6px hsla(0,0%,0%,0)!important;background-image:linear-gradient(to bottom,#5555,#2222)!important;padding-bottom:0!important;padding-top:0!important}menu,menuitem,.splitmenu-menuitem{-moz-appearance:none!important;color:#a5a5a5}#mainPopupSet menupopup menuitem:hover{background-color:#06a!important}#mainPopupSet menupopup menu:hover{background-color:#09f!important}#widget-overflow-mainView > .panel-subview-body > toolbarseparator,.PanelUI-subView menuseparator,.PanelUI-subView toolbarseparator,.cui-widget-panelview menuseparator,.cui-widget-panel toolbarseparator{margin:0}:root{--browser-background:#38383d--uc-light-bkgnd-color:#06a;--uc-active-color:#404040;--uc-text-color:#c3c6c9}tabbrowser tabpanels{background-color:var(--browser-background)!important}#context-navigation{background-color:transparent!important}#sidebar-box{background-color:hsl(240,7%,5%)!important}toolbarbutton[checked="true"]:not([disabled="true"]){color:inherit!important}#sidebar-header,.sidebar-splitter{border-color:hsl(240,3%,20%)!important}.sidebar-placesTree{color:#aaa!important}#mainPopupSet menupopup menuitem,#mainPopupSet menupopup menu{min-height:24px!important}.panel-arrowcontent,#placesContext,#alltabs-popup .arrowscrollbox-scrollbox,#contentAreaContextMenu,.textbox-contextmenu,#mainPopupSet menupopup,#bookmarks-menu-button menu menupopup{border:1px solid var(--uc-border-color)!important;background:var(--uc-dark-bkgnd-color)!important;color:var(--uc-text-color)!important}#alltabs-popup menuitem,#bookmarks-menu-button menu menupopup .subviewbutton{color:var(--uc-text-color)!important}#mainPopupSet menupopup,#mainPopupSet menupopup menuitem,#mainPopupSet menupopup menu,#alltabs-popup,#bookmarks-menu-button menu menupopup{-moz-appearance:none!important;border:none}.panel-subviews,#mainPopupSet menupopup,#bookmarks-menu-button menu menupopup > hbox,.search-panel-header.search-panel-current-engine,.search-panel-current-input,panelview,.PanelUI-subView{background-color:var(--uc-dark-bkgnd-color)!important}#BMB_bookmarksPopup{background-color:unset!important}#PanelUI-footer toolbarbutton:not(#PanelUI-quit):hover{background-color:red!important}#PanelUI-footer *,panelview toolbarbutton.subviewbutton .toolbarbutton-text,#downloadsFooter,#customizationui-widget-panel label,.autocomplete-richlistitem .ac-title,#widget-overflow-list toolbarbutton,#mainPopupSet menupopup menuitem,#mainPopupSet menupopup menu,#bookmarks-menu-button menu menupopup,#PopupSearchAutoComplete .autocomplete-tree.plain.search-panel-tree,#PanelUI-contents toolbarbutton,#sidebar-switcher-target,#alltabs_containersTab{color:var(--uc-text-color)!important}#alltabs-popup,#alltabs_containersMenuTab{border:none!important}#alltabs_containersMenuTab{-moz-appearance:none!important;background-color:transparent!important;list-style-image:none!important}#placesContext,#contentAreaContextMenu,#contentAreaContextMenu menupopup,#page-action-mainView,.textbox-contextmenu{-moz-appearance:none!important;background-color:var(--uc-dark-bkgnd-color)!important;border:none;list-style-image:none!important}#placesContext menuitem,#contentAreaContextMenu menuitem,#contentAreaContextMenu menu,.textbox-contextmenu menuitem{color:var(--uc-text-color)!important;-moz-appearance:none}#placesContext menuitem,#contentAreaContextMenu #context-navigation:not([hidden])~menu,#contentAreaContextMenu #context-navigation:not([hidden])~menuitem{min-height:20px!important;height:16px}#placesContext menuitem[disabled="true"],#contentAreaContextMenu menuitem[disabled="true"],#mainPopupSet menupopup menuitem[disabled="true"],#BMB_bookmarksPopup menuitem[disabled],.textbox-contextmenu menuitem[disabled]{color:#fff!important}#placesContext menuitem:hover,#contentAreaContextMenu menuitem:hover,#contentAreaContextMenu menu:hover,#contentAreaContextMenu menu[open],.textbox-contextmenu menuitem:hover{background-color:transparent;border:none}.autocomplete-richlistbox{background-color:var(--uc-dark-bkgnd-color)!important}#PopupAutoCompleteRichResult .ac-title,#PopupAutoCompleteRichResult .search-one-offs,.search-panel-input-value{color:var(--uc-text-color)!important}#PopupSearchAutoComplete{background-color:var(--uc-light-bkgnd-color)!important;color:var(--uc-text-color)!important}.search-panel-one-offs,.downloadsPanelFooterButton:not(:hover),#overflowMenu-customize-button:not(:hover){background-color:#fff!important}menuseparator{-moz-appearance:none;padding:0;border-top:0 solid #333;border-bottom:0 solid #048}menulist > menupopup > menuseparator{padding:6px 0 5px;border-top:1px solid red;border-bottom:none}menu,menuitem,menucaption{-moz-appearance:menuitem;-moz-box-align:center;color:#fff!important}menuitem[default="true"]{font-weight:400!important}menu[_moz-menuactive="true"],menuitem[_moz-menuactive="true"],.splitmenu-menuitem[_moz-menuactive="true"]{margin-bottom:0;border-radius:0;background-color:#06a!important;color:#e1e1e1!important;font-size:12px!important}menu[disabled="true"],menuitem[disabled="true"],.splitmenu-menuitem[disabled="true"],menu[_moz-menuactive="true"][disabled="true"],menuitem[_moz-menuactive="true"][disabled="true"],.splitmenu-menuitem[_moz-menuactive="true"][disabled="true"],toolbarbutton[disabled="true"]{color:#5a5a5a!important;display:none!important}menupopup menuitem[_moz-menuactive="true"][disabled="true"]{background-color:#646464!important}.chromeclass-menubar{padding-bottom:0!important;border-bottom:0 solid transparent!important}menuitem[default="true"]{font-weight:700!important}menuitem[label="(Empty)"],menuitem[label="(Empty)"][_moz-menuactive="true"]{-moz-appearance:none!important;color:#bf752b!important;background-color:transparent!important;min-height:20px!important;max-height:20px!important;border:1px solid red !imporant}#clearcache-menuitem{color:red!important}menu,menuitem,menucaption{-moz-appearance:menuitem;-moz-box-align:center;background:transparent!important}.panel-arrow[side="top"],.panel-arrow[side="bottom"]{opacity:0;position:relative;margin-left:10px;margin-right:10px}.panel-subviews,#mainPopupSet menupopup{border:0 solid red!important}.tab-label{white-space:nowrap;background:transparent!important}.tab-label-container{overflow:hidden;background:transparent!important}.bookmark-item > .toolbarbutton-text{margin-top:-1px!important}.bookmark-item{margin:-1px!important}.bookmark-item:not([container]):not([open="true"]) > .toolbarbutton-text{display:none!important}.bookmark-item:not([container]):not([open="false"]):hover > .toolbarbutton-text{display:block!important;background:transparent!important}.bookmark-item:not([container]):not([open="false"]):hover > .toolbarbutton-icon[label]:not([label=""]){margin-inline-end:0!important;background:transparent!important}.bookmark-item:not([container]):hover{margin-inline-end:0!important;background:#06a!important}

⋮ Tab Center Redux zDark Theme (Modern).css

This theme requires an addon created by Edouard Oger 
addons.mozilla.org/en-US/firefox/addon/tab-center-redux

Tab Center Redux 0.5.1 zDark Modern Theme
/* * * * * * * * * * * * * * * * * * * * * * * * * * * *
This theme requires an addon created by Edouard Oger 
addons.mozilla.org/en-US/firefox/addon/tab-center-redux
Place the Code below in the Custom Stylesheet section of the Add-on settings page.
* * * * * * * * * * * * CUSTOM CSS * * * * * * * * * * */

body.dark-theme{overflow:hidden!important;--tab-background-normal:210,9.289%,21.045%;--tab-background-pinned:210,100%,50.045%;--tab-background-active:210,9.289%,21.045%;--tab-background-hover:#31363b;--tab-border-color:#2b2f34;--menu-background:hsl(210,9.289%,21.045%);--menu-button-hover:#09f;--searchbox-background:#2b3034}#searchbox-input,#searchbox-icon{background-color:var(--searchbox-background);border:0 solid blue!important}#searchbox{border:1px solid #25282b!important;background-color:#2b3034}.tab.pinned.active{background-color:#006bbcff!important;border-top:0!important;border-right:0!important;border-bottom:0!important;border-left:1.5px solid #0080ff!important;border:solid;border-image:linear-gradient(to bottom,#2693ff 0%,#2693ff 53%,#0080ff 53%,#0080ff 100%);border-image-slice:1}.tab.pinned:not(.active){width:35px!important;height:24px!important;border-bottom:0 solid #2b2f34!important;border-top:0 solid #0664ac!important;border-left:0 solid transparent}.tab.pinned:not(.active):hover{height:24px!important;background-color:#006bbc!important;border-top:0 solid #0664ac!important}.tab.pinned.active:nth-of-type(1):hover{top:0!important}.tab.pinned.active:nth-of-type(2):hover{top:0!important}.tab.pinned.active:nth-of-type(3):hover{top:0!important}.tab.pinned.active:nth-of-type(4):hover{top:0!important}.tab.pinned:not(.active):nth-of-type(2):hover,.tab.pinned:not(.active):nth-of-type(3):hover,.tab.pinned:not(.active):nth-of-type(4):hover,.tab.pinned:not(.active):nth-of-type(5):hover,.tab.pinned:not(.active):nth-of-type(6):hover,.tab.pinned:not(.active):nth-of-type(7):hover,.tab.pinned:not(.active):nth-of-type(8):hover,.tab.pinned:not(.active):nth-of-type(9):hover{width:35px!important}.tab.pinned:not(.active):nth-of-type(1){padding-left:2.85px!important;border-left:1.5px solid #008dde!important;width:35px!important}.tab.pinned:not(.active):nth-of-type(2){border-top:0!important;border-right:0!important;border-bottom:0!important;border-left:1.5px solid #0080ff!important;border:solid;border-image:linear-gradient(to bottom,#2693ff 0%,#2693ff 53%,#0080ff 53%,#0080ff 100%);border-image-slice:1;padding-left:2.35px!important}.tab.pinned.active{padding-left:2.85px!important;background-color:#006bbcff!important;height:24px!important;width:35px!important}.tab.active.pinned:nth-of-type(1){width:35px!important}.tab.active.pinned:nth-of-type(2){width:35px!important;padding-left:2.55px!important}.tab.active.pinned:nth-of-type(3){background-color:ff0!important;width:34px!important;margin-left:0!important;left:0!important}.tab.pinned:not(.active):nth-of-type(7){border-left:1.5px solid #008dde!important;margin-top:1px!important;margin-bottom:1px!important}.tab.pinned:not(.active):nth-of-type(7),.tab.pinned:not(.active):nth-of-type(8),.tab.pinned:not(.active):nth-of-type(9),.tab.pinned:not(.active):nth-of-type(10),.tab.pinned:not(.active):nth-of-type(11),.tab.pinned:not(.active):nth-of-type(12){border-bottom:0 solid #2b2f34!important}.tab.pinned:last-child{border-right:0 solid #2b2f34!important;margin-top:0!important;position:relative}.tab.active:not(.pinned){background:linear-gradient(to right,#0099ffff 0%,#0099ffff 0.65%,#40464d 0.65%,#40464d 1.3%,#40464d 1.3%,#40464d 15%,#40464d 15%,#40464d00 100%);border-bottom:0 solid #2b2f34;border-left:0 solid #444b52;height:18px!important;color:#fff;padding-left:0!important;background-image:url(https://i.imgur.com/95km9Jy.png);background-repeat:no-repeat;background-position:0 0;background-size:auto 17px}.tab.active:hover:not(.pinned) /* Tab Hover */{background-image:linear-gradient(to right,#09f 0%,#09f 0.65%,#40464d 0.65%,#40464d 1.3%,#40464d 1.3%,#40464d 15%,#40464d 15%,#40464d 70%,#40464d 80%);border-bottom:0 solid #2b2f34;border-left:0 solid #444b52;height:18px!important;color:#fff;padding-left:0!important}.tab.active:hover:not(.pinned) > .tab-title-wrapper::after{--tab-background:var(--tab-background-normal);background-image:linear-gradient(to right,#0007 0%,#31363bff 100%);width:206.5px!important;height:18px!important;display:block!important;border-top:0 solid red!important;border-bottom:0 solid red!important;border-left:0!important;border-right:0!important;margin-top:0!important;top:0}.tab:not(.active):not(.pinned):hover{color:#fff;background:linear-gradient(to right,#09f 0%,#09f 0.65%,#0099ff88 0.65%,#0099ff88 1.3%,#0099ff88 1.3%,#0099ff88 15%,#006bbc 15%,#006bbc 80%);background-image:url(https://i.imgur.com/K50BLsP.png);background-repeat:no-repeat;background-position:0 0;background-size:auto 17px;background-repeat:no-repeat;background-position:0 0;background-size:auto 17px}.tab:not(.active) > .tab-title-wrapper::after{height:17px!important;display:block!important;border-top:0 solid #262f34!important;border-right:1px solid #272727!important;border-bottom:0 solid #272f34!important;border-left:0 solid red!important;margin-top:0!important;top:-1px}.tab:not(.active):hover > .tab-title-wrapper::after{--tab-background:var(--tab-background-normal);background-image:linear-gradient(to right,#ff000000 0%,#31363bff 90%,#31363bff 100%,#34393eff 100%);width:50px!important;height:18px!important;display:block!important;border-right:0 solid #34393e!important;top:0}#pinnedtablist .tab:last-of-type{border-bottom:none}#pinnedtablist .tab:last-of-type .tab-context{height:100%;top:0}#topmenu{display:flex;padding:6px 4px;height:32px;border-bottom:0!important;width:102%;background-image:linear-gradient(to left,hsla(0,35%,10%,0) 60%,hsla(0,0%,0%,0.1) 17%,transparent 0%),linear-gradient(to left,#2f3338ff 0%,#31363bff 70%,#262a2eff 100%);background-position:-7px 0;background-repeat:no-repeat;transition:background-position 190ms ease-out}#newtab-label{margin-left:0!important;white-space:nowrap;overflow:hidden;text-overflow:ellipsis;display:NONE;position:relative;left:6px!important;top:-1px;font-family:"Segoe UI";font-size:1em}#newtab{margin:0;margin-left:-1.8px;box-sizing:border-box;display:flex;flex:0 20 auto;width:20px!important;align-items:center;padding:2px 4.8px;text-shadow:var(--menu-button-text-shadow);border:0 solid #272b2e!important;background:#0099ff00;border-radius:100px}.tab-context{min-width:4.5px;height:100%;background-image:none!important;background-position:0 0;background-repeat:no-repeat;transition:background-position 999ms ease-out}#tablist-wrapper{height:94.35%!important;width:225px!important;flex-direction:column;overflow-y:visible;border:0 solid red!important;background:transparent}#pinnedtablist:not(.compact) .tab,.tab:not(.pinned):not(.active){display:flex;align-items:center;height:18px!important;width:200px!important;margin-left:0!important;margin-bottom:0!important;padding-right:0!important;border-bottom:0 solid #2b2f34;position:relative;background:linear-gradient(to right,#006bbcff 0.55%,#31363bff 0.6%,#31363bff 90%,#ffffff00 100%);background-image:url(https://i.imgur.com/41ZNQ4m.png);background-repeat:no-repeat;background-position:0 0;background-size:auto 17px}#pinnedtablist:not(.compact) .tab,.tab:not(.pinned),.tab.active:hover:not(.pinned){display:flex;align-items:center;height:18px!important;width:200px!important;margin-left:0!important;margin-bottom:0!important;padding-right:0!important;border-bottom:0 solid #2b2f34;position:relative}.tab.active:hover:not(.pinned){display:flex;align-items:center;height:18px!important;width:200px!important;margin-left:0!important;margin-bottom:0!important;padding-right:0!important;border-bottom:0 solid #2b2f34;position:relative;background-image:url(https://i.imgur.com/41ZNQ4m.png)}#pinnedtablist:not(.compact) .tab,.tab.pinned{display:flex;align-items:center}#tablist-wrapper #pinnedtablist{background:red!important;background-image:url(https://i.imgur.com/GlbnLiz.png)!important;background-repeat:repeat-y;background-size:1px 24px!important}#tablist-wrapper.shrinked #tablist .tab-meta-image,#tablist-wrapper.shrinked #pinnedtablist:not(.compact) .tab-meta-image{background:none!important;height:20px!important;width:20px!important;margin-left:.5px}.tab-icon{width:16px;height:16px;padding-top:1px;padding-bottom:1px}.tab-close,.tab-close:hover{position:absolute;display:block;width:24px;height:17px;top:0;bottom:0;right:0;margin:auto;background-image:linear-gradient(to right,#ff000000 0%,#ffff0000 100%)!important;background-position:center;background-repeat:no-repeat;background-size:16px;border-top-right-radius:50px;border-bottom-right-radius:50px;opacity:0;z-index:0!important}.tab:hover > .tab-close{opacity:0}.tab-close:hover{border-radius:var(--border-radius)}