Moving KDE’s styling into the future

One of the major issues with KDE’s styling system is the fact that over the year, it has accumulated four ways of styling applications – which makes themeing and changing aspects of the default theme far more cumbersome than it should be. In fact, with the current version of KDE, it’s effectively impossible to consistently theme the entire KDE desktop, as several parts of it, like Kirigami applications, only inherit parts of the theme you’re applying. It’s a bit of a mess, and KDE is well aware of this.

This problem is not new; we already identified it several years ago. Unfortunately, it also is not easy to solve. Some of the reasons it got to this state are simply inertia. Some things like Plasma’s SVG styling were developed as a way to improve styling in an era where a lot of the technologies we currently use did not exist yet. The solutions developed in those days have now existed for a pretty long time so we cannot suddenly drop them. Other reasons are more technical in nature, such as completely different rendering stacks.
↫ Arjen Hiemstra

These different rendering stacks form the core of the problem, as they can’t use the same rendering code for everything. Currently, KDE tries to address the problem through a compatibility layer to tie everything together, but it’s not perfect, it has to be maintained, and it means they’re not utilising their rendering stacks to their fullest potential. The solution KDE is working on is called Union.

However, there is another option, which is to take a step back and realise that we actually may not even want to share the rendering code, given that they are quite different. Instead, we need a description of what the element should look like, and then we can have specific rendering code that implements how to render that in the best way for a certain technology stack.
↫ Arjen Hiemstra

Basically, an input layer will interpret file formats with style descrpitions, while an intermediate layer consists of a library that converts that interpretation into a more abstract description of what needs to be rendered. The final output layer then uses the data from the intermediate layer to tell the rendering stacks what to do. By standardising on the input format, say CSS, it’ll be much easier to impement themes or make changes.

This effort is still far from done, but they’re making good progress. It ties into the Plasma Next initiative, which is, as the name implies, an effort to make changes to Plasma’s default look and feel.