Customize: Ensure that "Change" button appears when there are only 2 themes.

Fixes defect introduced in [35535].

Props igmoweb.
See #34549.
Fixes #35081.

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git-svn-id: https://core.svn.wordpress.org/trunk@35907 1a063a9b-81f0-0310-95a4-ce76da25c4cd