Linux dark themes have historically been a pain, styling-wise... I can't be sure without setting up the exact same gtk config and so on in a VM (which is tricky at the best of times, as behaviour can also depend on gtk and gnome versions), but if the issue was limited to that button, I'm hopeful today's nightly (with the fix from bug 1637652) is better - can you doublecheck once nightlies are available? Thank you!
Bug 1637295 Comment 4 Edit History
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Linux dark themes have historically been a pain, styling-wise... I can't be sure without setting up the exact same gtk config and so on in a VM (which is tricky at the best of times, as behaviour can also depend on gtk and gnome versions, icon themes, as well as other user configuration), but if the issue was limited to that button, I'm hopeful today's nightly (with the fix from bug 1637652) is better - can you doublecheck once nightlies are available? Thank you!