You need to turn off your compiled css if you are still developing, but this is where you change the "white":
#rt-mainbody-surround {
background-color: #FFFFFF;
border-bottom: 4px solid #E3D84B;
box-shadow: 0 0 35px rgba(0, 0, 0, 0.

;
}
You can add an image there also if you wanted to.
It's worth your while using FireFox/FireBug so you can actually see the code and what to edit and where.