My Thunderbird userChrome for square tabs + some minor UI tweaks

userChrome.css

/*
 * Thunderbird userChrome.css
 *
 * WARNING: This style was designed for TB version 60 and will likely not work
 * with more recent versions (see comments)
 * 
 * it's meant to be used with the default 3 panel split layout
 */

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);


/*
 * this changes the font for ALL UI elements
 * does not affect content (=emails in the viewing pane)
 */
* { 
	font-size: 8pt !important;
  	font-family: "Verdana" !important; 
	/*font-size: 10pt !important;
  	font-family: "Segoe UI Semibold" !important; */
} 

#dirTree-splitter, #sidebar-splitter, #threadpane-splitter,
#results-splitter, #compose-toolbar-sizer, #attachment-splitter,
#additionalFolderPaneSplitter, #threadpane-splitter,#calendarDisplayDeck, #today-pane-splitter {
    border: 0 !important;
}

#folderpane_splitter {
    min-width: 0 !important;
    width: 0 !important;
    max-width: 0 !important;
    border-left: 2px solid #f0f3f7 !important;
    border-right: 2px solid #f0f3f7 !important;
    margin-right:0 !important;
}
/*
 * the following deals with tabs:
 * 
 * 1. make them square
 * 2. Blend the active tab with the toolbar background
 * 3. Subtle background highlight for the hovered tab
 * 4. Slightly reduce the hight
 * --------------------------------------------------------------------
 */


#tabs-toolbar {
    margin-top: 10px !important;
    margin-left: 10px !important;
}

#tabs-toolbar .tab-text {
	font-size: 8pt !important;
	font-weight: bold !important;
}

/*
#tabs-toolbar .tab-background-start,
#tabs-toolbar .tab-background-end{
  display: none !important;
}

#tabs-toolbar .tab-background-middle{
  margin: -4px -2px !important;
  background: transparent !important;
}


#tabs-toolbar .tabmail-arrowscrollbox {
  margin: 0px !important;
}

#mail-toolbox{
  box-shadow: 0px -1px 0px threedshadow inset !important;
}

#tabs-toolbar .tabmail-tab {
  padding: 2px !important;
  max-height: 26px !important;
  height:26px !important;
}

#tabs-toolbar .tabmail-tab:not([first-tab]) {
  padding: 2px !important;
}

#tabs-toolbar .tab-text {
	font-size: 8pt !important;
	font-weight: bold !important;
}

#tabs-toolbar .tab-text:not([selected]):-moz-window-inactive {
  color: rgba(0,0,0,0.5) !important;
}

#tabs-toolbar .tabmail-tab:hover:not([selected]) {
  background: #d7d8da !important;
}

#tabs-toolbar .tabmail-tab[selected]{
  background: -moz-dialog !important;
  border: 1px solid threedshadow !important;
  border-bottom: none !important;
}
*/
 
/*
 * --------------------------------------------------------------------
 * end of tabs section
 */



#messengerWindow[sizemode="normal"] #navigation-toolbox {
  margin: 0px 0px 0px !important;
}

/*
 * background for the folder tree. normally white, but since we zebra-ize
 * it, i set it to color #2 and odd rows to white. You can do this the other
 * way around, of course, if you prefer a white folder tree background 
 */
#folderTree {
	background-color: #F0F3F7 !important;
    border-right: 1px solid #A9B7C9 !important;
}

/*
 * zebra-ize the folder tree 
 * (alternating row background colors
 * add some minor padding 
 * --------------------------------------------------------------------
 */
 

#folderTree > treechildren::-moz-tree-row(odd) {
	border: 1px solid transparent !important;
  	background-color:white !important;
}

#folderTree > treechildren::-moz-tree-row(selected) {
	border: 1px solid transparent !important;
  	background-color: #C1DBFC !important;
}

#folderTree > treechildren::-moz-tree-row(hover) {
	border: 1px solid transparent !important;
  	background-color: #FBE986 !important;
}

/*
 * adjust the row height to the slightly increase font size
 */
#folderTree > treechildren::-moz-tree-row {
	height:24px !important;
	min-height: 24px !important;
}

#folderTree > treechildren::-moz-tree-line {
    visibility: visible !important;
}

/*
 * bigger font for the folder tree
 */
#folderTree > treechildren::-moz-tree-cell-text {
	font-size:9pt !important;
	/*font-family: "Segoe UI Semibold" !important;*/
}

/*
 * some padding left and right (looks better imho, otherwise the)
 */
#folderTree > treechildren::-moz-tree-cell {
	padding: 0 8px !important;
}
/*
 * end of folder tree section
 * --------------------------------------------------------------------
 */


#threadTree {
    border-left: 1px solid #A9B7C9 !important;
    border-bottom: 1px solid #A9B7C9 !important;
    padding: 0 !important;
}
/*
 * alternating row colors in the message index pane
 */
#threadTree treechildren::-moz-tree-row(odd) {
	border: 1px solid transparent !important;
  	background-color:#F0F3F7 !important;
}

#threadTree treechildren::-moz-tree-row(selected) {
	border: 1px solid transparent !important;
  	background-color:#C1DBFC !important;
    margin: 0 !important;
}

#threadTree treechildren::-moz-tree-row(hover) {
	border: 1px solid transparent !important;
  	background-color:#FBE986 !important;
    margin: 0 !important;
}

#threadTree treechildren::-moz-tree-row {
	height:22px !important;
	max-height: 22px !important;
	padding-top: 1px !important;	/* this might be font dependent? */
    margin: 0 !important;
}

#threadTree treechildren {
	font-size:8pt !important;
	/*font-family: "Segoe UI Semibold" !important;*/
}


/*
 * make group headers stick out a bit more prominently
 * (give them a darker background)
 */
#threadTree treechildren::-moz-tree-row(dummy){
	background: #A0B0C9 !important;
	border: 1px solid transparent !important;
	margin-bottom: none !important;
    margin: 0 !important;
}

/*
 * tweaks for the message pane header
 * make it white, add some padding
 */
header-view-button-box {
  padding: 0px !important;
  border: 1px solid rgba(0,0,0,0.25);
}

#msgHeaderViewDeck {
  background-color: -moz-dialog !important;
  border-bottom: 0 !important;
}

/* flatify header (remove additional bottom border) */
#msgHeaderView {
    border-bottom: 0 !important;
}

#msgHeaderViewDeck label {
  	font-family: Verdana !important;
  	font-size:9pt !important;
}

#expandedsubjectBox .headerValue {
	font-size:10pt !important;
}

#msgHeaderViewDeck label#CompactHeader_collapsedsubjectlinkBox, #CompactHeader_collapsed1LsubjectBox .headerValue, #CompactHeader_collapsed2LsubjectBox .headerValue {
	text-decoration: none !important;
	margin-bottom: 5px !important;
	margin-top: 0 !important;
	-moz-appearance: none !important;
  	font-size:10pt !important;
  	line-height: 100% !important;
  	padding-top: 0 !important;
  	padding-bottom: 0 !important;
  	min-height: 12pt !important;
  	height: 12pt !important;
  	max-height: 12pt !important;
  	vertical-align: top !important;
}

/*
 * the message preview pane itself
 */
#messagepanebox {
  margin: 5px 0 0 0 !important;
  background-color: -moz-dialog !important;
  padding: 0 !important;
  border-left: 1px solid #A9B7C9 !important;
  border-top: 1px solid #A9B7C9 !important;
  border-bottom: 0 !important;
}

/* the message container (the part that displays the message, without header area) */
#messagepane {
	margin:5px 10px !important;
	border:1px solid #ccc !important;
	padding: 0 10px !important;
	background-color:white !important;
	box-shadow: 0 0 2px 0px rgba(0,0,0,0.35);
}

#expandedtagsRow { 
	display: none !important;
}

#item-description { 
}

#content-frame {
    border-left: 1px solid ThreeDDarkShadow;
    border-right: 1px solid ThreeDLightShadow;
    min-width: 10px;
    min-height: 10px;
    height: 400px;
	font-family: Consolas,monospace !important;
	font-size: 12pt !important;
} 
/*
 * Minor UI tweaks for the thunderbird-conversations addon. You can delete this 
 * section if you do not use that addon. Or leave it here; it does not touch 
 * anything else.
 */
@-moz-document url(chrome://conversations/content/stub.xhtml)
{

	/* changes background color of the message body*/

	body
	{
		background-color: #FFF !important;
	}

	/* eliminates the box shadow */

	#conversationHeaderWrapper
	{
		-moz-box-shadow: none !important;
		box-shadow: none !important;
	}

	/* changes attributes of the conversation header */

	#conversationHeader
	{
		padding: 4px 10px !important;
		background-image:none !important;
		min-height: 33px !important;
		max-height: 33px !important;
	}

	/* 
	 * the subject in the message header. Center it vertically, adjust the 
	 * height of the subject bar.
	 */

	.subject
	{
		font-weight: bold; !important;
		padding-top: 0px !important;
		padding-bottom: 12px !important;
		margin-top: 0px !important;
		margin-bottom: 12px !important;
		font-size: 14px !important;
	}

	/* 
	 * more subtle (= brighter) border around the message. Remove the box
	 * shadow and rounded corners.
	 */

	.message
	{
		border: 1px solid #ccc !important;
		-moz-box-shadow: none !important;
		border-radius: 0 !important;
		box-shadow: none !important;
	}

	#folderpane-title {
	    font-size: 1.2em !important;
	    font-weight: bold !important;
	}
}

I do no longer use Thunderbird at all and have not kept track of all the changes in it for the last couple of years. So I have no idea what change in TB is responsible for the breakage. According to some reports, the Style has been working mostly in TB 77, 78, but no later.

However, Supernova looks somewhat interesting, but claims that it's fundamentally rewritten in many areas. No wonder the style breaks. Maybe I'll take a look when I find time.

Are userChrome.css supported at all in 115?

Are userChrome.css supported at all in 115?

Yes, it's supported.