My GTK theme makes my filer background a different color; how do I make it white again?

Put something like the following in your ~/.gtkrc-2.0 file:
Note: to prevent this file from being overridden when changing themes use ~/.gtkrc.mine
instead

style "rox" = "default"
{
  bg[NORMAL] = "#ffffff"
  bg[ACTIVE] = "#ffffff"
}
widget_class "*Collection*" style "rox"

Other widget_class backgrounds you may like to change include:

• GtkToolbar - the toolbar at the top of the window
• GtkMenu - menus

but remember that this will affect all GTK applications.

How do I set a background image for ROX filer windows?

This is similar to the previous question. Put something like the following in your ~/.gtkrc-2.0 file:
Note: to prevent this file from being overridden when changing themes use ~/.gtkrc.mine
instead

pixmap_path "/home/yourhome/images"
style "rox" = "default"
{
  bg_pixmap[NORMAL] = "roxbg.png"
}
widget_class "*Collection*" style "rox"

Further reading

See the GTK Theming Tutorial for more information.