fix(shiny-preset): Improve Sass variable support (#1134) #2924

Job	Run time
routine / routine	4m 46s
R-CMD-check / setup	12s
website / pkgdown	5m 53s
R-CMD-check / macOS-latest (4.4.2)	2m 21s
R-CMD-check / windows-latest (4.4.2)	3m 8s
R-CMD-check / ubuntu-latest (devel)	2m 38s
R-CMD-check / ubuntu-latest (4.4.2)	2m 12s
R-CMD-check / ubuntu-latest (4.3.3)	2m 28s
R-CMD-check / ubuntu-latest (4.2.3)	2m 40s
R-CMD-check / ubuntu-latest (4.1.3)	2m 22s
R-CMD-check / ubuntu-latest (4.0.5)	2m 35s
Deploy to shinyapps.io	2m 33s
	33m 48s

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