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')