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.
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