I did some cleanup of the toolkit/themes/windows/global/menu.css for bug 1759510, and as part of that work I removed a few !important overrides of menu styling in that file.

Additional rule cleanup was uncovered during the review process that needs to be addressed: The !important color override for this menubar rule should be removed per Gijs (copied from his review comment):

"Hm, it seems to me like https://searchfox.org/mozilla-central/rev/997a56b018662e2940c99bbaf57a6ac9d1aa5422/browser/themes/windows/browser-aero.css#182-184 should trump https://searchfox.org/mozilla-central/rev/997a56b018662e2940c99bbaf57a6ac9d1aa5422/browser/themes/windows/browser.css#52-55 but it doesn't, purely because of the ordering of the rules (browser-aero gets used before browser.css ).

I would suggest dropping the !important here and fixing the ordering issue. Before we removed all the includes, browser-aero was at the bottom: https://searchfox.org/mozilla-esr91/rev/72dd08bbf4dd60917bce655e0de7673e9c0bcba2/browser/themes/windows/browser.css#753 .

Hopefully that means putting it back there doesn't cause significant issues (though it'd be worth doublechecking if we adjusted anything else when we moved that %include to an @import .

I would suggest that we could add the stylesheet in a Windows-only ifdef underneath chrome://browser/skin/, which points to browser.css because of how chrome URLs work - https://searchfox.org/mozilla-central/rev/997a56b018662e2940c99bbaf57a6ac9d1aa5422/browser/base/content/browser.xhtml#25 .

Alternatively, we could just put merge it completely into the Windows browser.css - at this point it's only 300-ish lines - IIRC originally it was not always used (only on Windows > XP, back when we ran on WinXP!) but we've not supported XP for yonks now, so I think the separation has lost its usefulness."