Created attachment 111527 [details] The headerbar selected (with slight transparent blue highlight) - still darker color. background-image: linear-gradient(to bottom, #556068, #475057); This makes things unreadable, and goes against the "light" colored "Breeze" theme being...light colored. Can we make the background color closer to white? I've enumerated a _lot_ of widgets with glade; this is the only one that seems 'wrong' out of the set.
Created attachment 111528 [details] Headerbar widget without selection, for actual color - its at the bottom row by itself.
Created attachment 111529 [details] Remmina with an unreadable + button ; it blends in
Created attachment 111532 [details] Same test-case with Adwaita theme; notice, its brighter
Created attachment 121946 [details] Fix for dark headerbars when using gtk3-nocsd You appear to be using gtk3-nocsd. The dark headerbar makes sense when not using gtk3-nocsd, but looks very out of place when using it. To fix this, do the following: 1. Install "gtk3-nocsd". 2. Select "Breeze" for both the KDE and GTK themes. 3. Copy the attached "gtk.css" file into "~/.config/gtk-3.0/". 4. Log off and on (not necessary if you already had gtk3-nocsd installed). This will result in the following visual changes in GTK3 CSD applications: 1. Headerbars are now the same color as the window background. 2. Button and text color is changed appropriately. 3. Titles in headerbars are now hidden to avoid duplicating the titlebar text. 4. Subtitles are vertically centered within the headerbar. 5. The rounded top corners are no longer drawn on the headerbar. This fix was tested using KDE neon with Plasma 5.16.4. I don't know how well it works with earlier versions.
Created attachment 121950 [details] screenshot showing the difference with and without my CSS hack
It's not accurate to say that nothing else dark; the headerbar color is simply the titlebar color, because a headerbar is a titlebar that happens to have a bunch of app-specific UI controls in it. There were many bugs in the implementation prior to Plasma 5.17, but in that (upcoming) release, the colors are much more consistent and appropriate (See Bug 356006). If you don't like the fact that the headerbar is dark, you'll be able to choose a light color scheme like Breeze light, which will result in all headerbars and titlebars being light, not dark. If you don't like headerbars at all and wish that affected apps had separated titlebars and toolbars, that's not something we can affect in Breeze-GTK, and you'd be better off using grk3-mushrooms or gtk3-nocsd or any of the other similar packages.