How do I theme the Nautilus background image?

1. Open the file /usr/share/themes/MyTheme/gtk-3.0/apps/nautilus.css (Requires root permission).

2. Add this line:

   NautilusWindow * {background-image:url("bg.jpg");}

   where bg.jpg is the background image you want to display.

3. Run nautilus -q to kill all open file browsers.

4. Run nautilus.

This will apply background to all classes present inside NautilusWindow.You can identify the classes where you don't want to apply background and add background-image:none;
Example:

NautilusWindow .sidebar,
NautilusFloatingBar,
NautilusWindow .sidebar .view {
    background-image: none;
}

The part of the theme you want to alter is .view, so in nautilus.css include the following section:

NautilusWindow *.view {
  background-color: @your_predefined_color_here;
}

From my own experimentation on 12.04.1 LTS, I've found that it is possible to define your own background-color property in here with a hexadecimal RGB value, but you cannot set a background-image using -gtk-gradient.

Thus, you probably won't be able to set a background image, but you can at least change the color.

Note: if you don't want this to affect your sidebar, follow the last part of ignite's answer, but use background-color instead of background-image unless you also set that further down in your theme file.

I got the idea to theme .view partially from this Arch Linux forums thread.