Starting in 5.9, block themes are not compatible with (do not support) Customizer; rather, they use the Site Editor. Viewing installed themes in Customizer, this commit adds an overlay message to alert users and give them a way to activate the block theme. Clicking on the "Activate" button activates the block theme and redirects back to the Appearance > Themes interface, where the user can then enter the Site Editor for customization.
Non-block themes are not affected by this change and continue to work in Customizer.
Follow-up to [41648], [41893], [52279].
Props antonvlasenko, costdev, hellofromTonya, jffng, joyously, noisysocks, poena, shaunandrews.
Fixes#54549.
Built from https://develop.svn.wordpress.org/trunk@52371
git-svn-id: http://core.svn.wordpress.org/trunk@51963 1a063a9b-81f0-0310-95a4-ce76da25c4cd