Clean up <ThemeProvider> and various CSS related stuff #2594

Job	Run time
Build	2m 58s
Unit test	2m 57s
Static code analysis	3m 6s
Print commit stats	1m 17s
Type definitions test	3m 3s
Build samples	27s
HTML test (${{ format('{0}/{1}', matrix.shard-index, matrix.shard-count) }})	0s
Merge test results	4s
Merge test snapshots	4s
	13m 56s

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