Customizer info section:

* Only show the theme info when doing a live preview of another theme.
* Replace with helper text about what you are customizing and what the Customizer is for.
* Fix focus styling.

props celloexpressions, westonruter. fixes #28550.

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git-svn-id: http://core.svn.wordpress.org/trunk@28822 1a063a9b-81f0-0310-95a4-ce76da25c4cd