How to change a background based on the category

I’m attempting to change the background of my pages for different categories on my site. For all of the posts in the art category I’m using:

body.category-art {
background-color:#000;
background-image:url(https://asvpart.com/new/wp-content/uploads/2011/07/bkgrnd_gallery.jpg);
background-repeat:no-repeat; background-size: 100%;
}

This is a link to the page

But its not working correctly. It seems to work for the main page of the category, but not for all of the posts that fall under that category.

[duplicate: https://www.ads-software.com/support/topic/targeting-a-specific-category-to-replace-background-images?replies=4 – please continue with your original thread]