Document dark/light/gray "magic" more explicitly?

[maelle]

Via https://github.com/r-lib/pkgdown/pull/1682#issuecomment-872978930 cc @apreshill

I see the variables vignette mentions that fg and bg are used but I think it'd be clearer if it stated that it means the resulting colors are not grays but a mix of fg and bg.

red_theme <- bslib::bs_theme(version = 4, fg = "red", bg = "yellow")
vanilla_theme <- bslib::bs_theme(version = 4)

bslib::bs_get_variables(red_theme, c("dark", "light", "gray-500", "border-color"))
#>         dark        light     gray-500 border-color 
#>    "#FF3300"    "#FFE600"    "#FF8000"    "#FFB200"
bslib::bs_get_variables(vanilla_theme, c("dark", "light", "gray-500", "border-color"))
#>         dark        light     gray-500 border-color 
#>    "#343a40"    "#f8f9fa"    "#adb5bd"    "#dee2e6"

^{Created on 2021-07-29 by the reprex package (v2.0.0)}