[Docs] Missing guide on how to create variants of Sweet

[LJNeon]

This theme is written with a combination of sass, svgs, and small scripts that seem like they would make it rather easy to create new variants. However there is no documentation on how someone would go about doing that.

This seems worth the effort to me because it would create the opportunity for users to submit pull requests with new variants that could then be made available to everyone. And if people just wanted to create a personalized variant they could easily do so. This would require minimal effort on the maintainer's side, after any necessary changes are made and the documentation is written only reviewing and merging pull requests for new variants would be necessary.

[LJNeon]

I've put some effort into figuring this out on my own, and while I've figured out some things that seem to work (node 10 & node-gyp 5), I'm unable to perfectly replicate the built versions found on gnome-look. Whitespace and calculated colors/opacities seem to be ever so slightly off sometimes.

It also seems like there's an install.sh script that isn't included in the repository, and I have yet to successfully replicate all of the rendered images.