You can most likely extract the background from the apk, which is just a zip archive* with a different extension.
http://sourceforge.net/projects/cren...er3/cr3-3.1.2/
* You can rename the apk with a zip extension and then open it with your favourite zip program. I use 7-zip which allows you to right-click > Open Archive without renaming first.
ETA:
Here is the background from Cool Reader and another I use: