Customize: Fix rendering issues in theme browsing when opening theme details...

Customize: Fix rendering issues in theme browsing when opening theme details or performing search in Safari.

Props sayedwp.
See #37661.
Fixes #42052.

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git-svn-id: https://core.svn.wordpress.org/trunk@41782 1a063a9b-81f0-0310-95a4-ce76da25c4cd