Theme
A theme provides tokens through the single theme slot. Declare kind: "theme" and client.slots: ["theme"]; no host half is needed.
Register a component returning a React-managed <style> element with a stable ID. Unmounting it removes its CSS and restores the product fallback without a page reload.
Supply both palettes
Timon's app uses :root for the dark palette and .light for the light palette. Supply both. This is the Timon plugin convention, distinct from the light/dark attribute used by this documentation website.
The complete plugin-theme template covers surfaces, shadows, component tokens, brand colours, calendars, syntax highlighting, grid and scrollbar tokens. A partial palette is valid: the permanent timon-floor cascade layer supplies omitted values. Unlayered plugin declarations override the floor for the tokens they define.
Do not override --timon-topbar-h, --timon-nav-w or --timon-browser-w. These are measured runtime layout values. Replace geometry through a layout plugin.
Selection and access
Installation alone does not apply a theme. The client mounts the theme selected in the user's Appearance preferences alongside the product fallback. Other installed themes stay available in the gallery without painting.
The viewer must also have access to the plugin/theme permission target. A theme the viewer cannot use does not expose its client bundle to that viewer. Test switching, disabling and both colour modes.