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.
Project, tag, Today, and Inbox theme settings can also use separate background
images for light and dark mode. The Darken/lighten background image for better
contrast slider and the Blur background image slider are shown when at
least one background image is set. The blur slider defaults to 0px. The
Disable colored background tint setting is shown when at least one mode has no
background image and still uses the app's tinted background.
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.