Feature: Add global dark / light theme toggle (persisted & accessible) #3013
Description
Summary
Add a global dark / light theme toggle to the site UI so users can switch between light and dark modes. The preference should persist (localStorage), affect the whole site (body/class), be keyboard accessible, and respect prefers-color-scheme on first load.
Why
Improves readability and reduces eye strain for night-time users.
Matches modern UX expectations for documentation websites.
Increases accessibility and customization options.