How to edit CSS of Category List Items

My goal is to add unqiue background-colors to my category lists.

See attachment.

For example, I opened up Chrome DevTools and added a red color gradient to "Open" category. I see that the <ul class="disucss-categories-list unstyled"> has <li class="list-item" so I added it here.

How and where do I make this permanent and unique for categories called "Open" "Lacrosse Talk" "Stringing Showcase" and "String Community". Where are these files located and how do I do it?

I am not experienced with coding, so a helpful explaination will mean alot.

thanks,