Twenty Thirteen: minor adjustments to bundled "star" header image. Props joen.
Twenty Thirteen: better selector for widget titles. Fixes #23580.
Twenty Thirteen: improve styling for RSS widget title, fixes #23571.
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.
Twenty Thirteen: instantly preview header text color changes in the Theme Customizer. Fixes #23585, props SriniG.
Twenty Thirteen: first pass at cleaning up unneeded browser prefixes, props Jayjdk. See #23582.