An example plugin to read, copy and change: twenty animated backdrops behind the chat in the desktop app, with frosted-glass panels: one per project or per session. /custom-theme list names them. Fireworks go off over the theme when Claude says "merged"; /fireworks sets them off by hand.
Check that your Claude client is supported and that you installed the plugin in the workspace or project you have open. In Claude Code, run /reload-plugins or reopen the session.