Accessibility: Administration: Apply a unified design to all secondary buttons...

Accessibility: Administration: Apply a unified design to all secondary buttons across all color schemes.

This ensures that secondary buttons have an appropriate color contrast in the alternative color schemes.

Props melchoyce, ryelle, audrasjb, ate-up-with-motor, afercia, sabernhardt.
Fixes #48585.
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git-svn-id: https://core.svn.wordpress.org/trunk@46615 1a063a9b-81f0-0310-95a4-ce76da25c4cd