Changing the background color of one section of a page

Hello, I am trying to change the background color of a certain area on my webpage, developed using the WordPress engine. The section contains expanding/collapsing content so I need the color to not be fixed at a certain height and width like some gradients are, I just need the color to start at right underneath the seperation bar and end when it touches the other seperation bar. But when one of the terms is clicked, I would like that color to expand along with the area if that makes sense.

(Link to the page I am speaking about)

I would like the area that contains “Terminology” to have a background of #f6f6f6 that expands if a term is clicked. I have tried using both section and div tags with specific IDs but to no avail. Any idea on how I would go about this? Thank you