usr /share /caja /patterns is the folder that contains the images you can see as a caja background.
To change the background:
open caja, from Edit menu , select Backgrounds and Emblems and drag the pattern on white caja background.
Since some of these patterns are really ugly, I replaced them with others made by me.
To replace them you should open caja as administrator.
If you put a small image, this is repeated to fill all the space and you can create geometric patterns
Alternatively you can choose a color, even choosing a custom color.
terracotta.png
dark mate
Brushed_metal.png
stucco.jpg
Italian speaking link: http://www.istitutomajorana.it/forum2/Thread-Sfondi-per-caja