Desktop app: support custom themes / accent colors (parity with CLI custom theme system)

Problem: The desktop app offers only light/dark (+ high-contrast). With most modern apps shipping visually identical near-black dark modes, Claude Desktop windows are indistinguishable at a glance from other apps across multiple monitors — color is how many users recognize windows without reading title bars.

Request: Let users re-color the desktop app chrome (sidebar / chat background / accent), ideally reusing the existing ~/.claude/themes/*.json custom-theme system the terminal CLI already supports, or at minimum an accent-color picker. The Code-appearance syntax themes (e.g. Catppuccin) already prove out per-surface theming — extend it to the app UI.

Environment: Claude Desktop on macOS 15 (Darwin 24.6), Claude Code engine 2.1.215.

(Searched existing issues — all current custom-theme issues concern the CLI/TUI theme system; none request desktop chrome theming.)