Themes screen: Various IE8 adjustments and fixes.

Merges [27007] to the 3.8 branch.

props morganestes, matveb.
fixes #26541.


git-svn-id: https://develop.svn.wordpress.org/branches/3.8@27008 602fd350-edb4-49c9-b593-d223f7449a82
Responsive improvements to submenus in the toolbar.

Merges [27006] to the 3.8 branch.

 * Remove .ab-sub-wrapper rules in the 782px media query, as they're duplicates of the rules set on line 161.
 * Reposition the arrows for submenus at responsive sizes where the menus are taller.
 * Set the submenu position to static at smartphone sizes, so it's not positioned outside of its parent (and thus offscreen).

props undergroundnetwork, iammattthomas.
fixes #26720.


git-svn-id: https://develop.svn.wordpress.org/branches/3.8@27009 602fd350-edb4-49c9-b593-d223f7449a82