CSS for Site Layouts in Jekyll With Minima

[njsch]

Apologies if this is not the most entirely appropriate way of (or platform for) asking this. I am a vision impaired (completely blind) Jekyll user and I have thematically applied the Minima default to two of my Github Pages sites, namely njschmidt.id.au and eduvis.com.au.

The problem is simply that, even after consulting the readme, I still don't really understand how to apply CSS integrations within the context of Jekyll themes in terms of layout overrides. It tells you that it is possible but seems a little potentially lacking on how to go about it. My sites currently look rather plain with very little in the way of fonts and colours. If I was using plain HTML then I could simply include the CSS but with Jekyll, it's different. Can someone help point me in the right direction here? I'm a bit of a kinesthetic learner, so If someone showed me at least once then I'd probably get it.

Thanks in advance.