Do you mean the logo twitter having a white background, or the text after it having a light brownish? colour?
.twr-timeline has this style:
background:#FBF8F3 none repeat scroll 0 0 !important;
that's right at the end of like the 5th line (one big long line of code). Then there's some border-radius afterwards to make the corners rounded.
I saw a lot of importants in there...