Dark mode gone after reload

When selecting dark mode, this user setting seems to be persisted nowhere. After refreshing the page, the app is displayed in light mode again.

Would be nice if the user choice would be persisted.

Bonus: Initially, the browser/OS setting could be taken into account.

No files or tests are named. Start by tracing the dark-mode selector and the initialization path used after a page reload; confirm how the current theme is chosen and where a user preference could be restored. Done means the selected theme survives refresh, with browser or OS preference handling only if the bonus is included.

Written by the indexing model from the issue text.