I am trying to update the way the shopping cart looks on my

I am trying to update the way the shopping cart looks on my site. If I change to a default WP theme it looks okay, but when I use my theme and any of the MP themes, it's all squished together. I tried updating the modern.css file but nothing is happening. I looked at the source code and discovered that none of my changes are being recognized. When I uploaded fiesta.css, I don't even get that theme as a drop-down on the Store Settings tab.

Thank you in advance for any help on this issue.