Few More Customizations for Default Dark Theme.css

/* Tab bar customization */
:root:-moz-lwtheme-brighttext {
 --chrome-background-color: #424242 !important;
 --chrome-color: #8f8f8f !important;
 --chrome-secondary-background-color: #363636 !important;
 --toolbox-border-bottom-color: #2e2e2e !important;
}
.tabbrowser-tab[visuallyselected="true"]:-moz-lwtheme {
	color: #dddddd !important;
}	

/* Fixing tabGroups menu */
#tabGroups-tabview-button .toolbarbutton-text {
	max-height: inherit !important;
}
toolbar {
	min-height: 37px !important;
}
toolbar .toolbarbutton-1 {	
	padding: 3px 2px !important;
}

/* Container tab borders */
[data-identity-color="blue"] {
 --identity-tab-color: #7193F7 !important;
 --identity-icon-color: #7193F7 !important;
}
[data-identity-color="green"] {
 --identity-tab-color: #71F78A !important;
 --identity-icon-color: #71F78A !important;
}
[data-identity-color="orange"] {
 --identity-tab-color: #F2DE73 !important;
 --identity-icon-color: #F2DE73 !important;
}
[data-identity-color="pink"] {
 --identity-tab-color: #F771E7 !important;
 --identity-icon-color: #F771E7 !important;
}

:root:-moz-lwtheme {
  --tab-line-color: none !important;
}

To hide the border on top of the active tab

.tabbrowser-tab:not([usercontextid]) > .tab-stack > .tab-background > .tab-bottom-line {
  background: linear-gradient(to right, transparent 10%, #F2DE73 10%, #F2DE73 90%, transparent 90%) !important;
  height: 2px !important;
}

To set a colored border(bottom) on all tabs

toolbar {
	min-height: 37px !important;
}
toolbar .toolbarbutton-1 {	
	padding: 3px 2px !important;
}

No need anymore.

Refer https://gist.github.com/dumindu/dabf0c0d436ab68f6684999ebef0a0d1 for the latest code.