Hello Samuel,There's an idea for the future of theming in Plasma 6 that would use CSS to define a "universal theme" that would then be consumed by our QQC2, Plasma, and GTK theming to keep them all in sync automatically. See https://phabricator.kde.org/T13467. This is one of the options presented there, but to my knowledge the only one with a prototype implementation, made by Arjen Hiemstra (CCd). If you're interested in pursuing this, you might want to get in touch with him about it.
Nate
