A challenge for integrating with Kvantum is that it isn't great for theming QtQuick due to bugs in our existing creaky theming infrastructure, as well as development direction preferences on the part of Kvantum's maintainer.

One thing we have planned and want to work on during the Plasma 6 lifecycle is a new unified theming system that can apply to all KDE and Qt apps, GTK apps, and Plasma. The idea is to have a new theme that can be directly consumed by Qt's Qstyle (for QtWidgets apps), KDE's QtQuick desktop style (for desktop QtQuick apps), KDE's Plasma style (for Plasma), as well as KDE's GTK theme. Essentially we would end up with a new theming engine and each of the existing themes we have would consume those themes. This would replace the current approach where the C++ QStyle is the central source of truth and our QtQuick desktop style pulls content from it, while our Plasma and GTK themes are totally separate and have to be changed manually.

The new proposal is in fact not unlike how Kvantum already works, but it's not rally made for easy upstreaming and it also uses SVG as the basis for its themes. We'd like to build our own thing and investigate using CSS as the basis for themes.

Needless to say, this is not happening for Plasma 6.0. :) But I'm hoping we can get it done sometime in the next year or three.