If you’re following KDE Plasma development, you’ve most likely run into something called Union, a project KDE is working on to unify their various ways of theming their applications. The problem KDE is facing right now is that after so many decades of development and changes in how people want to develop applications, they ended up with various different ways of writing applications, each with their own theming method. The end result has been that for a while now, theming on KDE is kind of broken.
Broken in what way? Most long-time KDE users will be aware that ever since KDE 4, the KDE shell (Plasma using SVG for theming) and KDE applications (QtWidgets using QStyle for theming) use separate theme engines. While this has always been annoying, it’s at least manageable in that most theme designers tended to create both a Plasma SVG theme and a QStyle theme that matched. However, things got more complicated when KDE introduced QtQuick, its modern way of creating applications with QML. QtQuick has its own theme, qqc2-desktop-style, to make QtQuick applications look and feel like Breeze, KDE’s current theme.
Not only do all of these have to be kept in sync manually, QtQuick applications also do not properly inherit all the elements of the QStyle theme you set, leading to many modern KDE applications looking broken when using a non-default theme (and the same applies when using Kvantum; it also cannot properly theme QtQuick applications). In other words, there is currently no way to theme the entire KDE desktop for a consistent look, and if you try, many applications will simply look broken.
Union is KDE’s answer to this set of problems. Union is a new style engine that takes CSS and processes it into consistent themes for both QtWidget and QtQuick applications. It’s quite flexible, and can potentially even be extended to generate GTK themes from that same CSS. Sadly, since the KDE Pasma shell SVG stuff is entirely different, it won’t be styled by Union, but KDE might simply retire the SVG stuff entirely and move the Plasma shell to QtQuick’s qqc2-desktop-style to address that issue.
Union has been in development for a long time, as it’s a difficult effort, but progress is definitely being made. KDE is currently already at the stage where they’re adapting the current Breeze QStyle to better match the Union Breeze’s style, to make the future transition from the separate QStyle/qqc2-desktop-style to the unified, single Union Breeze as seamless as possible. These changes are currently available for testing in the master branch, and will be part of Plasma 6.7 or 6.8.
As a KDE user who likes to have a more classic, late ’90s theme, but who also values consistency above all else, Union is something I’m very much looking forward to. While it certainly won’t fix every single issue right away, it will definitely address the biggest issues with theming on KDE. I’m incredibly happy that KDE’s developers still consider theming and user choice and agency over what pixels appear on their screen important enough to undertake an effort like Union.
After all these year, I might go back to KDE.
I think I first used in very early SuSE linux (after they switched from being a Slackware fork). And… have been a Gnome user in between… for about 3 decades now.
(I put all clones like Unity in the GNOME bucket)
KDE had been stable, and decided not to break too much from norm. I would still prefer something like GNOME 2.0, but those days are long gone.