feat: backgrounds with globals (#848)

* feat: backgrounds with globals

* fix

* fix: format

* fix: test

* fix: changeset

* fix: changeset

* fix: background filling the space

* fix: resizing crash

* fix: lock backgrounds when overriden

* fix: formatting

* put behind a feature flag

* fix: updated changeset

* docs: add information about new feature flag for backgrounds

* fix: jotai warning

* fix: formatting

* fix: crash and lint

* fix: panel not filling height

* feat: add example of using backgrounds for switching the theme

* fix: formatting

* fix: update for clarity on backgrounds change