KDE has many apps and many ways to render them. Over the years, different toolkits and styling tricks accumulated, so some apps can all feel like they belong to different worlds. Union is a new system meant to fix that without throwing away what already works.
What Union does?
Union is a middle layer that takes one style description and translates it so each app can render the same look using whatever toolkit it uses. It’s not trying to replace existing systems. Instead, it helps them cooperate so themes look consistent across the desktop.
How it’s built — three simple layers
- Input (what designers write)
- Reads styles written by designers.
- Supports familiar formats so designers can keep using tools they know.
- Started from Plasma’s SVG themes and moved toward CSS-like inputs because CSS is easier for designers.
- Intermediate (the shared description)
- A neutral, toolkit‑agnostic model that describes UI elements, states, sizes, and colors.
- Think of it as the common language that captures what the final look should be.
- It must be expressive enough to represent real designs but not so specific that it favors one rendering method.
- Output (how apps draw it)
- Backend plugins convert the intermediate model into drawing commands for each toolkit (Qt Quick, Qt Widgets, SVG, etc.).
- Each toolkit renders the style using how it makes the most sense for it.
Why this approach?
- Keeps existing work: No need to throw out SVGs, widget styles, or other systems.
- Flexible: Toolkits keep using their best rendering methods.
- Designer-friendly: Uses familiar authoring formats and will support good tooling.
- Supports creativity: Doesn’t block wild or experimental themes.
Challenges Addressed by Union
- Different rendering technologies can’t share code directly.
- Designers need authoring formats that are easy to use and test.
- The intermediate model must stay neutral — not leak backend details.
- Compatibility matters: themes and apps should keep working as the system evolves.
Unifying Themes Progress so far
- Focused first on Qt Quick, since many modern KDE UIs use it.
- They used Breeze (KDE’s dominant style) as a test and looks very close to the traditional versions.
- A comparison tool shows controls side-by-side to spot differences.
- Prototype outputs for Qt Widgets exist; a qstyle-compatible output is next on the list.
What it won’t do
- It won’t replace every styling system or force a single renderer.
- It won’t stop designers from making unusual themes.
- It won’t be a black box — the goal is to be usable by designers and maintainable by developers.
Roadmap — next steps
- Expand CSS-style input support and handle legacy SVG themes better.
- Improve the intermediate model so that it fits real-world needs.
- Finish and harden Qt Widgets/qstyle output.
- Build better tools and docs so theme authors can start quickly.
- Target an experimental Union release with Plasma 6.7, then grow from user feedback.
Why this matters to you For users: more consistent look and feel across apps. As for designers: easier ways to author themes that work everywhere. For developers: less duplicated work and easier testing. For KDE’s future: a flexible system that lets toolkits evolve without breaking themes.
If you want to try or help, watch for Union updates around Plasma 6.7. It’s a practical step toward making theming easier and more consistent while keeping KDE’s creative spirit.
