That option should be available for this theme soon, I’m currently finishing an update for it. For now, the background image can be changed from the CSS around line 1072 –
/*
Widgets
---------------------------------------------------------------------------------------------------- */
/* Slider Wide (Full Width)
--------------------------------------------- */
.slider-wide {
clear: both;
}
Or it can be changed from the regular .slider
section right below that.
I can help you with this if you need it done now and don’t want to wait for the update (which should be done soon).