but it is even easier load your forums in your browser and right click on the element you wish to change. Depending on your browser the next part may differ, in chome you click inspect in other browsers is developer something
this opens an in browser code inspector.
You can select the html code you want and it highlights on the page it also shows you the css for this element along with the file and line number associated with it.
as you can see in the screenshot
the class that controls the background color is found in the colours.css file on line 131 .bg2
its actually two classes for the colors as they alternate both bg1 and bg2