There are an awful lot of system preferences. Since you don't want to be specific, you can march through each and use the "restore to default" button, if there is one. Many don't have one, including the Desktop & Screen Saver.
You could try creating a new user, then reboot into that user account. Then make note of the various settings you are interested in. Then reboot back into the first account and apply them.
Or just be more specific.