The Customizer and Theme Installer open in full overlays that need to receive
focus. Also, keyboard navigation should be constrained within the overlays. Using
CSS `visibility` to hide all the content except the overlays, makes them the only
available and focusable content and allows browsers to handle focus natively.
See #29158.
Fixes#33228, #27705.
Built from https://develop.svn.wordpress.org/trunk@38520
git-svn-id: http://core.svn.wordpress.org/trunk@38461 1a063a9b-81f0-0310-95a4-ce76da25c4cd