@media screen and (max-width: 33.75em)
index_dark.css:2054
.forumposts .windowbg2 {
background-color: #fafafa;
This comes directly from Chrome desktop, just resizing the screen in order to trigger the relevant media query.
Just be a bit more verbose, so that you can learn:
1) open chrome (or firefox for the matter)
2) resize the window until the background becomes white,
3) open the built-in developer tools (i.e. right click "inspect" on the white area)
4) browse up the document markup structure presented in the development tool window until you find a background color in a media query ( @media something).
In that case you have to go up until you reach:
<div class="post_wrapper windowbg2">
that is the "problem", change the css according to your needs in the relevant file and you are done.
BTW: I'd suggest you to remove the min-width: 750px from the body, so that people with small smartphones will not have an horizontal scroll bar.