For some colors of the site it is necessary to determine the CSS selector and then use CSS to change them. For instance the selector for the form in the left sidebar is
HTML Code:
#loginwithajaxwidget-3
So to make the background white you would use the following.
HTML Code:
#loginwithajaxwidget-3 {
background: #ffffff;
}
I use the Firebug addon for Firefox for determining selectors.