Update the Themes screen, merging THX into core.

 * Name: THX38
 * Description: Update the Themes screen with a new design and experience.
 * Tags: visually-focused, bigger-screenshots, fast, mobile-friendly, backbone-driven
 * Author: matveb, shaunandrews, melchoyce, designsimply, shelob9
 * URI: http://wordpress.org/plugins/thx38/

fixes #25948



git-svn-id: https://develop.svn.wordpress.org/trunk@26141 602fd350-edb4-49c9-b593-d223f7449a82