How would i go about changing/adding a new backround?

Before this toybox update I installed a custom background from gmod.org. Is there any way to get rid of this backround and change it back to a default just so I know in the future?
Also , how would I go about making my own custom background and background songs?

custom backgrounds are found in the materials folder, the original background in in the GCF file, you can make one by saving it and replacing it with your own (in the folder, not in the GCF)