Themes screen: Various IE8 adjustments and fixes.

Merges [27007] to the 3.8 branch.

props morganestes, matveb.
fixes #26541.

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git-svn-id: http://core.svn.wordpress.org/branches/3.8@26885 1a063a9b-81f0-0310-95a4-ce76da25c4cd