sub theme?

[LinhUMD]

Is there a "for not drupal devs" guide for sub-themes? There are some minor css things we'd like to adjust for links and not quite sure where to go for the sub-theming of this.

In trying to test just the basics, having the sub-theme, no real changes. It shows up but when set as default, it throws a "The website encountered an unexpected error. Please try again later."

I've created the two yml files and I think with what I need (I believe you can see it, I originally forked it from here).

Ultimately, I know we could just directly edit since this is not really active, but it'd be good to know for the future.