Accessibility: Restore the primary buttons original background color for...

Accessibility: Restore the primary buttons original background color for alternate color schemes after [46241].

Props david.binda, audrasjb, azaozz.
See #34904.
Fixes #48396.

Built from https://develop.svn.wordpress.org/trunk@46575


git-svn-id: https://core.svn.wordpress.org/trunk@46372 1a063a9b-81f0-0310-95a4-ce76da25c4cd