I now understand what you want, which is as indicated by bibblebub, ......Wallpaper., Not a Theme. Consequently, I have moved from my Desktop comp to Laptop which is loaded with Windows 7 and have tested matters.
Click Start > Control Panel > Personalisation > Desktop Background > (Picture location...Drop down Menu Box) > Solid Colours > Choose colour > Save Changes.
However, as I am thinking that already you have been through all the steps as listed above, I am back to thinking that perhaps a bit of malware has entered your computer.
I am able to appreciate your reluctance to try Malwarebytes for fear of causing a conflict with Norton. Such would be the case if you you installed the Premium Edition of Malwarebytes which runs in real time whereas the free edition only scans on demand.
Check to make sure that your SuperAntiSpyware is Not running in real time and therefore parallel with your Norton, because that will certainly be a conflicting 'set-up'.
Hans.