styles: refactor dark theme switching for markdown elements #7162

Job	Run time
Unit Test (ubuntu-latest, lts/*)	37s
E2E test and link check (ubuntu-latest, lts/*)	7m 17s
Lint (ubuntu-latest, lts/*)	1m 2s
Proselint (ubuntu-latest, lts/*)	44s
	9m 40s

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