Editor: Fix invalid navigation overlay patterns

Fixes the following two patterns:

- Navigation Overlay: Add explicit black text and link colors so the default overlay pattern remains readable even when the active theme uses white text.
- Overlay with site info and CTA: Fix invalid block attributes to prevent the block from breaking.

Follow-up to [61609].

Props isabel_brison, mikachan, onemaggie, westonruter, wildworks.
See #64589.

git-svn-id: https://develop.svn.wordpress.org/trunk@62114 602fd350-edb4-49c9-b593-d223f7449a82