You can edit the gtk-main.css or nautilus.css files.
I recommend you to try this under your ~/.themes and not system-wide. You may either change the background color of nautilus only or the base theme as a whole, which affects a lot more programs. In this example I will change the background color in the default Ubuntu Theme "Ambiance" (in Ubuntu Gnome use the folder Default instead) from white (#ffffff) to a light grey (#D8D8D8). Feel free to pick any other color.
For comparison how it looks like in the beginning:
Create a copy in ~/
Create your ~/.themes directory and copy the original Ambiance theme to it.