Clean up <ThemeProvider> and various CSS related stuff #2596

Job	Run time
Build	2m 43s
Type definitions test	3m 4s
Build samples	30s
Print commit stats	1m 6s
Unit test	2m 58s
Static code analysis	2m 48s
HTML test (${{ format('{0}/{1}', matrix.shard-index, matrix.shard-count) }})	0s
Merge test snapshots	4s
Merge test results	6s
	13m 19s

Provide feedback