Add 'todo' field to dark mode settings #2577

Job	Run time
NodeJS for prerender	1m 8s
CSS for HTML themes	1m 19s
PDF/LaTeX backend	3m 31s
Julia 1.6 - ubuntu-latest - x64	4m 0s
Julia 1.6 - macos-latest - x64	9m 54s
Documentation: html	1m 14s
Julia 1.6 - windows-latest - x64	16m 45s
Documentation: pdf	2m 53s
Julia 1 - ubuntu-latest - x64	3m 32s
Julia 1 - macos-latest - x64	8m 4s
Linkcheck: online tests	1m 54s
Julia 1 - windows-latest - x64	5m 13s
Julia nightly - ubuntu-latest - x64	4m 29s
Linkcheck: Documenter manual	9m 52s
Julia nightly - macos-latest - x64	9m 22s
Julia nightly - windows-latest - x64	6m 55s
Julia 1 - ubuntu-latest - x86	4m 10s
Prettier (JS code formatting)	5s
	1h 34m 20s

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