Amending CSS in a plugin

I’ve purchased the wp directory box theme via Envato and I’m using its inbuilt customiser to customise a site, all the while referring to their comprehensive online documentation. However there are basic changes I want that are not covered, changes which require amendments to the CSS.

The Author has provided the file path to the code and I’ve even referred to youtube for guidance on how to achieve what I want, namely change rectangles with curved edges to full circles: A matter of changing the radius I think?

The problem for me is that the code in the videos I’ve watched doesn’t seem to bear any resemblance to the code I’m seeing, for these element. I also want to make sure any changes I do make are responsive.

So, I’d be grateful for some guidance. Here’s a link to their demo page