Twenty Seventeen: Fix style issues with gallery image links

* Ensures dark color scheme has proper box shadow and hover/focus styles.
* Make sure links are correct width.

Props afercia, laurelfulford.

Fixes #38969.

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git-svn-id: https://core.svn.wordpress.org/trunk@39358 1a063a9b-81f0-0310-95a4-ce76da25c4cd