Customize: If the return URL for Close button is a page added to the...

Customize: If the return URL for Close button is a page added to the Appearance menu by a deactivated theme, fall back to the Themes screen.

Props JarretC, Jean-David, SergeyBiryukov.
Fixes #46686.
Built from https://develop.svn.wordpress.org/trunk@46754


git-svn-id: https://core.svn.wordpress.org/trunk@46554 1a063a9b-81f0-0310-95a4-ce76da25c4cd