Reapply appearance-tools theme support from #3846 #875

Job	Run time
Core running from build / Core running from build / Linux	2m 11s
Core running from src / Core running from src / Linux	1m 58s
Core running from build / Core running from build / Windows	4m 12s
Core running from src / Core running from src / Windows	3m 46s
Core running from build / Core running from build / MacOS	3m 38s
Core running from src / Core running from src / MacOS	3m 49s
Gutenberg running from src / Gutenberg running from src / Linux	7m 16s
Gutenberg running from build / Gutenberg running from build / Linux	7m 28s
Gutenberg running from build / Gutenberg running from build / Windows	10m 55s
Gutenberg running from src / Gutenberg running from src / Windows	10m 27s
Gutenberg running from build / Gutenberg running from build / MacOS	11m 7s
Gutenberg running from src / Gutenberg running from src / MacOS	10m 27s
Leave WordPress Playground details	4s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 17m 18s

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