docs/wiki/3.09-Theming.md
Custom theming is supported on desktop (Electron) only. You load a single custom CSS file that overrides or extends the app's styles.
The app ships with several built-in themes that you can select from the settings:
You can also color-code individual projects and tags with custom primary, accent, and warn colors.
styles.css in your user data folder (not in a subfolder). The app loads it at startup and injects it into the renderer.styles.css behavior.