Make --pluto-schema-types-color a brighter color in dark mode

[abhro]

Make sure it stands out against a dark background, and make it a very slightly dimmer version of --pluto-output-color in dark mode.

Fixes #3557.

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/abhro/Pluto.jl", rev="dark-mode-color")
julia> using Pluto

[fonsp]

Thanks!!