one easy way you could achieve this is by just adding the needed background in
body#bd
found in the template.css file.
once you do that the only places which remain would be ones under main and that have padding / margins….
so in say main you could add…
.main {background-color:#FFFFFF; }
this would mostly fill up most of the places with white except some which have padding / margin.
for example > the#ja-mainnav has a bottom padding of 10px. if you remove that…then it would align well with the slideshow and the gap in between would disappear.
Through this you would see background similar to the forum