Comment 16 for bug 1871644

I'm not quite familiar with how this works, I think Martin is the right person to ask about it.

I can dig if you want but I'd guess that this kind of stuff (the `:not(:-moz-lwtheme)` bits and such) is related: https://searchfox.org/mozilla-central/rev/9bc5dcea99c59dc18eae0de7064131aa20cfbb66/browser/themes/linux/browser.css#389-399