Hi Becca,
Thanks for the quick reply. I tried adding the code component. It indeed changes the background color, but unfortunately it only changes the color once I quite the fullscreen mode. Otherwise the background color remains unchanged.
However, I found a different solution with the polygons now. I added a polygone with a thick line width (e.g. 50) and its size specified as norm (2,2). This draws a nice frame around the screen and its color can be changed and updated by specifying the line color with a variable. 
Thanks a lot!