Twenty Fourteen: minor style adjustments for navigation and a new lighter pattern-dark graphic which makes easier to change the background color of the featured image content. Props iamtakashi.

Also update stylesheet tags. See #25946.
Built from https://develop.svn.wordpress.org/trunk@26272


git-svn-id: http://core.svn.wordpress.org/trunk@26177 1a063a9b-81f0-0310-95a4-ce76da25c4cd