Hey everyone! For those of you who are looking for info on how to make themes, I figured it out. Here is a link to my github repo. In there, pay attention to the icy-nord and icy-nord-darker themes.
In the non-diff versions of the files, my changes are right at the bottom of the file (i.e. they are based on the winternord theme and I added my css which overwrites the original css at the bottom).
There is a comment block somewhere towards the end which signifies where my changes start.
I used Stylus to test out the themes while I was working on them.