Twenty Fourteen: CSS fixes, props celloexpressions. Fixes #26188.

 * Remove unnecessary color layer in header nav (same as header background).
 * Fix site description padding with header shadow.
 * Create the MediaElements players' time rail and volume container backgrounds with a semi-transparent color so that it adjusts with the player background color if that's changed.
Built from https://develop.svn.wordpress.org/trunk@26579


git-svn-id: http://core.svn.wordpress.org/trunk@26469 1a063a9b-81f0-0310-95a4-ce76da25c4cd