This commit ports to core the changes to the classes that deal with theme.json code.
See #54336.
Props oandregal, spacedmonkey, noisysocks, hellofromtonya, youknowriad.
git-svn-id: https://develop.svn.wordpress.org/trunk@52049 602fd350-edb4-49c9-b593-d223f7449a82