Syncs the necessary changes from Gutenberg to allow setting site-wide background images using the top-level `styles.background` key in `theme.json`. Props ramonopoly. Fixes #61123. Built from https://develop.svn.wordpress.org/trunk@58222 git-svn-id: http://core.svn.wordpress.org/trunk@57685 1a063a9b-81f0-0310-95a4-ce76da25c4cd