waimus' Firefox CSS basic customization. Uses additional CSS file linked in the file.

userChrome.css

/*https://www.userchrome.org/*/
/*more stuff at https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome*/

@import url("./window_control_placeholder_support.css");  /*from: https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/window_control_placeholder_support.css*/
@import url("./tabs_on_bottom.css");  /*from: https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/tabs_on_bottom.css*/

/*
    Additional modification from external css above help to tweak the look better.

    i.e:
    tabs_on_bottom.css
    --uc-titlebar-padding

    window_control_placeholder_support.css
    --uc-window-control-width
    --uc-window-drag-space-width

    ---------------------------------------------------
    Make sure to all imported files are in the same folder as this folder. (~/.mozilla/firefox/$PROFILENAME/chrome/).
    Excuse the poor naming/commenting.

*/

/*  window control button direction  */
#TabsToolbar {
  direction: rtl;
}

/*  tabs button direction  */
#tabbrowser-tabs {
  direction: ltr;
  --tab-line-color: rgb(15, 15, 15) !important;
}

/*  private mode tab line colour  */
#main-window[privatebrowsingmode] #tabbrowser-tabs {
  --tab-line-color: #9400FF !important;
}

#main-window {
  --toolbar-bgcolor: rgb(15, 15, 15) !important;              /*  toolbar/navbar background colour  */
  --lwt-toolbar-vertical-separator: transparent !important;     /*  separator colour before the app menu button in navbar */
  --chrome-content-separator-color: rgb(30,30,30) !important;     /*  separator between the whole toolbar to the web view   */
  --lwt-background-tab-separator-color: rgb(30,30,30) !important; /*  inactive tab separator  */
}

/*  small bookmark toolbar  */
#PersonalToolbar {
  visibility: visible;
}

/*  tab minimal height  */
:root {
  --tab-min-height: 28px;
}

/*  active tab border  */
#navigator-toolbox {
  --tabs-border-color: rgb(30,30,30) !important;
}

/*  titlebar/tab bars background colour  */
:root:-moz-lwtheme{
  /*background-color: rgb(10, 10, 10) !important;*/
  background-image: linear-gradient(rgb(15,15,15) 6%, rgb(0, 0, 0) 12%) !important;
}

/*  active tab button  */
#tabbrowser-tabs:not([movingtab]) > #tabbrowser-arrowscrollbox > .tabbrowser-tab > .tab-stack > .tab-background[multiselected="true"]:-moz-lwtheme, #tabbrowser-tabs:not([movingtab]) > #tabbrowser-arrowscrollbox > .tabbrowser-tab > .tab-stack > .tab-background[selected="true"]:-moz-lwtheme {
    background-attachment: scroll, scroll, fixed;
    background-color: rgb(15, 15, 15);
    background-image: linear-gradient(rgb(15,15,15) 30%, rgb(20, 20, 20)) !important;
}

/*  url bar text  */
#urlbar-input {
    text-align: center !important;
}

/*  window button properties  */
.titlebar-min {
  order: 2 !important;
  margin-left: 6px !important;
  margin-right: 6px !important;
}

.titlebar-max {
  order: 1 !important;
}

.titlebar-close {
  order: 3 !important;
  margin-right: 6px !important;
}
/*Spacer before and after tabs*/
.titlebar-spacer[type="pre-tabs"] {
  width: 8px !important;
}
.titlebar-spacer[type="post-tabs"] {
  width: 8px !important;
}