I did some digging for you in the browser console. userChrome.css can't touch TST's content. TST is in a separate document (sidebar.html). I'm sure the tabs in TST aren't the actual browser tabs, but they're made to look exactly the same as before. Talk to TST's author, or modify the addon yourself, it must have a CSS file, the tabs are "li.tab" (<li> elements, tab class). The active one is "li.tab.active".
document.querySelector('#sidebar')._contentWindow[0].document.querySelectorAll('.tab.active')
Re:Extensions in Firefox 58
by CWuestefeld on Saturday November 25, 2017 @02:25PM
(#15777476)
Thanks. Actually, it turns out that the latest version contains a mini-editor for the appropriate file (whichever it is). And it includes a link [1] to further documentation about what to do with it.
[1] https://github.com/piroor/treestyletab/wiki/Code-snippets-fo...
Before FF57, I was able to set the font and background color for tabs depending on their status (current, inactive, unread, etc.), and that's what I want now. IIRC, this was actually a feature of Tab Mix Plus.
It's clear that this is still possible even without TMP, but requires putting code in userChrome.css. That would be fine with me, if I could just find any documentation on what element classes (or whatever would be the most appropriate selectors) I need to reference.