Theme Customizer: Add a 'previewing' notice by the theme name. Reduce padding...

Theme Customizer: Add a 'previewing' notice by the theme name. Reduce padding and size on section titles, add a border between section titles and content to better indicate the title as a click target. see #19910.

git-svn-id: https://core.svn.wordpress.org/trunk@20404 1a063a9b-81f0-0310-95a4-ce76da25c4cd