featured image area color

hello

I felt like the default size for the featured images in my theme was too big and overpowering, so I reduced it. Now, because the photo does not span the entire width of the page, the background color from the rest of the page is shown around the photo. Is there a way to get the part where the photo ends and the background begins to be colored black instead?

example:

https://www.virtuesofhealing.com/mission

theme:’nonprofit’ by ‘organic themes’

thanks for your help!