How to modify Featured Image "Bubbles" on Category Pages

Hello. I use a solid black background with my theme. On my category pages that list my blog posts, there are default white image “circles” or “bubbles” that use the featured image for the individual post.

My featured image is scaled and doesn’t encompass the full “bubble” leaving a lot of white space.

Is there a way to change this white circle to black? That will make the category pages much more thematically appropriate with my site.

Here is a link for your reference:

http://www.jerhow.com/category/themeparks/

Thanks so much!

– jerhow