feat(theme): Branded theming via brand.yml #2995

Job	Run time
R-CMD-check / setup	41s
routine / routine	4m 55s
website / pkgdown	4m 54s
R-CMD-check / macOS-latest (4.4.2)	2m 20s
R-CMD-check / windows-latest (4.4.2)	3m 48s
R-CMD-check / ubuntu-latest (devel)	3m 12s
R-CMD-check / ubuntu-latest (4.4.2)	2m 19s
R-CMD-check / ubuntu-latest (4.3.3)	2m 42s
R-CMD-check / ubuntu-latest (4.2.3)	2m 27s
R-CMD-check / ubuntu-latest (4.1.3)	2m 33s
R-CMD-check / ubuntu-latest (4.0.5)	2m 28s
Deploy to shinyapps.io	0s
	32m 19s

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