Go to Settings > Appearance > Application Style > Configure GNOME/GTK application style. The dropdown will be blank and GTK apps won't follow the system color scheme if you change it to Breeze Dark (in Settings > Appearance > Colors), for example. This problem is very old.
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For some reason the screenshot didn't upload, so here it goes:
This problem happens after a fresh install.
One thing that I forgot to mention is that this also makes the Adwaita to be loaded as a fallback theme, so GTK apps look inconsistent.
This is odd. I could have sworn this is supposed to auto-activate. @rdieter, @ngraham: do you have any ideas why this isn't working? Is there a setting we're supposed to set or something to force it?
Metadata Update from @ngompa: - Issue tagged with: experience
I haven't heard any reports of this upstream FWIW.
This happens in the latest release, 35, but previous ones are also affected. I tested in a VM right now:
This has been forwarded upstream as kde#446572.
Metadata Update from @ngompa: - Issue close_status updated to: Deferred to upstream - Issue status updated to: Closed (was: Open)