Themes: Normalize Windows backslashes in theme directory paths. #29064

Job	Run time
Core running from src / MacOS	2m 40s
Core running from build / MacOS	3m 23s
Core running from src / Windows	3m 50s
Core running from build / Windows	3m 58s
Core running from src / Linux	2m 21s
Core running from build / Linux	2m 30s
Slack Notifications	0s
Failed workflow tasks	0s
	18m 42s

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