Allow renderers to override `_any_color_fmt`. #2377

Job	Run time
NodeJS for prerender	48s
CSS for HTML themes	57s
Documentation: html	51s
PDF/LaTeX backend	1m 27s
Linkcheck: online tests	1m 0s
Documentation: pdf	57s
Linkcheck: Documenter manual	53s
Prettier (JS code formatting)	6s
Julia 1.6 - ubuntu-latest - x64	53s
Julia 1.6 - macos-latest - x64	1m 13s
Julia 1.6 - windows-latest - x64	9m 23s
Julia 1 - ubuntu-latest - x64	53s
Julia 1 - macos-latest - x64	1m 48s
Julia 1 - windows-latest - x64	1m 48s
Julia nightly - ubuntu-latest - x64	1m 5s
Julia nightly - macos-latest - x64	2m 0s
Julia nightly - windows-latest - x64	2m 9s
Julia 1 - ubuntu-latest - x86	1m 1s
	29m 12s

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