Theme Editor: Fix styling of child theme notice.

Props westonruter, melchoyce, ndiego.
See #24048.
Fixes #42491 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42155


git-svn-id: https://core.svn.wordpress.org/trunk@41986 1a063a9b-81f0-0310-95a4-ce76da25c4cd