I am new to Atahualpa--I just changed some of the settings so now the background color of my site is black instead of white. I am unable to change the background color of the forms (Login, comments, contact form). I changed the color under the Atahualpa settings, but it wasn't effective.

What version of atahualpa and wp?
What is your URL?

WP 3.0.4
Atahualpa 3.6.1
http://rhiannacooper.com

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

#loginwithajaxwidget-3

So to make the background white you would use the following.

#loginwithajaxwidget-3 {
background: #ffffff;
}

I use the Firebug addon for Firefox for determining selectors.