Trying to make a color background change

On my site I want to remove the background color from the registration form or just change it to #FFFFFF.

I am using Firefox Web Developer Tools and I see the entry I need to change (attached) but register does not appear to be a .css file. Maybe a .php file? In either case I cannot find it anywhere.

Under style menu in the Developer Tools it reads as follows…

#content .standard-form { Register#2 (Line 54)

-moz-border-radius:6px 6px 6px 6px;

background:none repeat scroll 0 0 #FFFFFF;

border-color:#CCCCCC #999999 #999999 #CCCCCC;

border-style:solid;

border-width:1px;

float:left;

margin:0;

padding:3%;

width:93%;

}

If I can find the css file to make my changes that is all I will need for now. Please see attached.

P.S. I had to resize the attachment because the last one failed. Hopefully this one will still make my point.