Clean up <ThemeProvider> and various CSS related stuff #2598

Job	Run time
Build	3m 14s
Build samples	27s
Static code analysis	3m 16s
Unit test	3m 21s
Type definitions test	6h 0m 15s
Print commit stats	1m 6s
HTML test (${{ format('{0}/{1}', matrix.shard-index, matrix.shard-count) }})	0s
Merge test snapshots	4s
Merge test results	4s
	6h 11m 47s

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