changing background color of all posts in a category

The client wants the change the background color on some pages on this site:

On the following category:

The client wants to do the following:
change the background to the regular white color in the background of all the Fun Pages (ffffff - 255/255/255) instead of the whitish green one that all the other pages have.

She only wants to do this to the posts in the fun pages category. For example, this is one of the posts whose background would color would need to be changed:

Can you please tell me how to do this?