So, tl;dr: I think there's a reasonably good chance that your ` ~/.config/gtk-3.0/gtk.css` file (and/or about:config prefs) was in fact the thing that was providing the colors for this sidebar; it was probably *not* your Firefox theme, though it's not surprising that someone (e.g. past-you) might set those theming constants up to look nice together and then forgotten which part was responsible for coloring which bits of the UI. Having said that, if you can find an old Firefox version that respects the Firefox theme colors for this sidebar instead, then I'm happy to be wrong and that would be interesting, and it would present an actionable rabbit-hole that we could go down to bisect builds to see if we can find out when/why that changed!
Bug 1908200 Comment 15 Edit History
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So, tl;dr: I think there's a reasonably good chance that your ` ~/.config/gtk-3.0/gtk.css` file (and/or an `about:config` pref like `ui.-moz-sidebar`) was in fact the thing that was providing the colors for this sidebar; it was probably *not* your Firefox theme, though it's not surprising that someone (e.g. past-you) might set those theming constants up to look nice together and then forgotten which part was responsible for coloring which bits of the UI. Having said that, if you can find an old Firefox version that respects the Firefox theme colors for this sidebar instead, then I'm happy to be wrong and that would be interesting, and it would present an actionable rabbit-hole that we could go down to bisect builds to see if we can find out when/why that changed!