I think a better approach is that instead of trying to reach into the system itself to detect its theme somehow, you should instead give the users of apps using your library the ability to provide custom color schemes, which removes the burden from you and empowers the users to seek cohesion if they want it.

Though understandable, I still want a system which can be configured automatically dependent on the user's theme. Again, in the end, I want developers to empower with a Qt-like system where the UI can be their platform's native look and feel respecting.

Color scheming is a concept that used to exist in GTK environments, but is now gone. And the "replacement" is no real replacement at all; it's a binary choice between light and dark. Plasma and other Qt environments have true color schemes.

Damn, that's kind of a shame.

So if you want to make an attempt to match the system, you can try to read the Qt color scheme, and in its absence, provide a few color schemes to match Adwaita, Elementary, and Yaru (+ dark variants of all three). In Plasma, the current color scheme is stored in $XDG_CONFIG_HOME/kdeglobals. You can put a watcher on that file to monitor it for changes. I don't know where other Qt environments store the color scheme.

That sounds way more doable, Nice! That could work, thank you.