I'm pretty sure you can, I've done it before. I just dont remember the path. I've changed all the icons this way but I'm not sure how to replace the background.
I can only make this remark. I have done hundreds of modifications to my phone over the several years. I remember each and every one and the vast majority I have since removed as many were worthless or I got tired of them.
But I remember how I did each and every one and could redo each and every one. The fact that you have to ask someone to tell you how to do this suggests to me you could be mistaken.