Twenty Thirteen: header area with no site title or tagline should still be clickable to home. Fixes #23549.

Fix also removes several `!important` flags that aren't necessary, relating to header text color.

git-svn-id: http://core.svn.wordpress.org/trunk@23492 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: instantly preview header text color changes in the Theme Customizer. Fixes #23585, props SriniG.

git-svn-id: http://core.svn.wordpress.org/trunk@23493 1a063a9b-81f0-0310-95a4-ce76da25c4cd