Customize: Give the WordPress logo a white background for dark mode browsers.

The favicon is transparent, which would normally look fine, but on a dark background it washes out.

Props DhrRob, audrasjb.
Fixes 49798.

Built from https://develop.svn.wordpress.org/trunk@47564


git-svn-id: https://core.svn.wordpress.org/trunk@47339 1a063a9b-81f0-0310-95a4-ce76da25c4cd