Admin: Add CSS fallback values in button and media views.

Since button and media view CSS can sometimes be enqueued by extenders in contexts where either `wp-base-styles` are not present or the `body` wrapper class is omitted, these variables should use fallback values.

Reviewed by audrasjb.
Props huzaifaalmesbah, wildworks, mukesh27, noruzzaman, joedolson, sabernhardt, audrasjb, shailu25.
Fixes #64840.

git-svn-id: https://develop.svn.wordpress.org/trunk@62154 602fd350-edb4-49c9-b593-d223f7449a82