I'm trying to change the color of the panels that pop up when using certain widgets on my web app. We have cable systems in the highest capacity tier colored white so they stand out the most on our map. However, as the background of the Legend and the left-hand layer list in the Jewelry Box theme are white, this color is lost. See image below for what I'm trying to edit.

I just need to know which file I can find the right attribute to change.