In T13467#254255, @ngraham wrote:

Another option that we've been discussing recently:

1. We make a universal theme of some sort (using CSS, QML, or something else; exact format TBD)
2. We make a QStyle that is essentially a Kvantum-style theming engine that consumes the universal theme and applies it to Widgets-based apps
3. We make a QQC2 style that consumes the universal theme and applies it to QtQuick software, including (optionally) Plasma, without going through QPainter or the QStyle at all
4. We keep the existing Plasma QQC2 style that knows how to consume existing SVG-based Plasma themes, for compatibility's sake
5. We add a user-facing setting for apps to allow people to choose whether to have:
   • Their apps use the universal theme
   • Their apps use a different QStyle, such as Lightly or Kvantum
6. We add a user-facing setting for Plasma to allow people to choose whether to have:
   • Plasma use the same universal style as their apps
   • Plasma use a different universal style from the one being used for their apps
   • Plasma use an SVG-based Plasma 5 theme, which we deprecate in Plasma 6 but continue to support
7. In Plasma 7, we sunset support for SVG-based Plasma themes as well as generic QStyles (including Kvantum!) and only support the new universal theme--whatever it is we come up with