Gnome does not expose this at all (except for the built in dark mode) and they actively discourage any kind of theming. Gnome 47 will finally support accent colors, which they reluctantly implemented after it became a freedesktop standard and most distros were patching it in anyway.
I will caveat this by saying I haven’t worked with GTK and don’t know how well-tooled it is in this regard. If GTK doesn’t offer any/many parameterized values, then that’s on GTK, not app devs. They’re pretty well supported in macOS/AppKit, iOS/UIKit, and Android/Compose though and should be a cornerstone of any modern UI framework.
You can have a logo for your app that is coloured green. Then the user is using an all-green theme that happens to match the shade of your image and the logo is basically invisible.
That’s just easiest counter-example I could come up with.
I mean even OSX has had accent colors for years, ffs.
> It's mostly just changing colors and some border radius.
This can still break apps though, as it's impossible to test all possible color themes to see if the app has enough contrast with all of them.
> After all this is desktop linux, people tend to gravitate to it because they want to be able to tweak things.
This is what I particularly don't get though. Compared to GTK this seems to be more limited. Granted, GTK does not officially support custom style sheets and lately they have become harder to set, but the option is there and people have been making themes that completly change how some widgets look like. All of that seems fundamentally impossible here.
> I mean even OSX has had accent colors for years, ffs.
There is a xdg portal to set accent colors (from a limited testable set of colors) since some months. I wonder if libcosmic supports that or if you're forced to manually set a theme.
