Better visual focus syling in customizer, theme preview, and attachment details modals.

props celloexpressions.
fixes #29147.

Built from https://develop.svn.wordpress.org/trunk@29608


git-svn-id: http://core.svn.wordpress.org/trunk@29382 1a063a9b-81f0-0310-95a4-ce76da25c4cd