Is there a guide or documentation on how Skins work? I have some override settings for the VIX Dark skin that I want to keep (after a package update for example)

Also what do you guys use for skin design? I came across this project which I built from source - https://gitlab.com/technic93/e2designer but I can't seem to be able to use it to modify ViX Dark skins due to the nestled nature of the skins.