it looks like it is just local files in the root of your repo - not a gem based theme. If you know the css you want to edit it should be in the sass folder which I think was inside assets, and edits in there should work no problem.
If you don’t know which css needs to be edited, that isn’t really a jekyll issue. I’d also assume that the theme creator put that space there for a reason and editing one thing may change the behavior of other things. You could try asking on the theme’s repo >>issues.