tests: Add `318-navbar-colors` #526

Job	Run time
routine / routine	3m 1s
R-CMD-check / setup	48s
website / pkgdown	3m 15s
R-CMD-check / macOS-latest (4.4.2)	3m 36s
R-CMD-check / windows-latest (4.4.2)	5m 49s
R-CMD-check / ubuntu-latest (devel)	3m 7s
R-CMD-check / ubuntu-latest (4.4.2)	2m 57s
R-CMD-check / ubuntu-latest (4.3.3)	2m 56s
R-CMD-check / ubuntu-latest (4.2.3)	3m 9s
R-CMD-check / ubuntu-latest (4.1.3)	4m 3s
R-CMD-check / ubuntu-latest (4.0.5)	3m 3s
	35m 44s

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