Unique <body> ID's

I'm designing a layout for a client of mine and run into a problem.

I shrunk down the .main part of the shop down to 960px. Done this also for the .header and .footer

Now I'm left with a white background. I added a custom background to this body, untill now all is fine.

No the actual problem i'm having. The background shows up in the administration section of the website in the TinyMCE editor.

How can I give the cutomer part a custom background, and leave the admin area untouched?

I tried making a custom class for the customer part of the shop, but this was not so successful.

Can anyone help me out?