OK, some more information since no one has come back yet.
I found this thread on styling the rss widget but the proposed solution of using:
#sidebar .rsswidget {background-color:#FFF;display:block;}
…does nothing except fill in the <h2> and the elements. Here is a screen grab of what I have so far.
I have tried:
#sidebar .rsswidget
#sidebar ul.rsswidget
#sidebar li.rsswidget
div.rsswidget
#sidebar div.rsswidget
.rsswidget
…and quite a few other variations but try as I might, I simply cannot get the whole block to have a white background because it is contained in a generic-named div called class=widget.