Fix CSS clashes on the Network Themes page.

Merges [26919] [26923] to the 3.8 branch.

props SergeyBiryukov.
fixes #26513, #26517.

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git-svn-id: http://core.svn.wordpress.org/branches/3.8@26829 1a063a9b-81f0-0310-95a4-ce76da25c4cd