Theme JS file for quick config

[mattbag]

I Think it takes nothing to make a theme config file, to allow people to quickly add their own to the theme shell.
We can start with the colour palette, but eventually even the layout would be a nice addition, while using inline styles and css grid.
Looking forward to other opinions! Cheers

[stefanjudis]

I think that's an excellent idea. 👍

Let's maybe make this issue only around colors first to keep complexity low and take it from there then?