Enable user to set default color scheme

I have to admit, while I think the PR https://github.com/JuliaDocs/Documenter.jl/pull/1456 is interesting and useful, I'm not sure it's a great idea by default. I think at least having an optional feature to override the default color-scheme would be beneficial.

For example, many documentation sites are not optimally set-up for dark-theme first (if the user has the OS set to a dark theme).

For example, take a look at https://jump.dev/JuMP.jl

Similarly, the Makie docs look better with a light-default, as the animations and plots use a white background by default