large input box background color

It's coming in from this in wood.css:

[css]
.titled_box_content, .fancy_box, .fancy_box_title, .pullquote, .toggle_frame, .minimal_table td, .textfield, .password, .textarea, #s, #comment {
background: url(wood/black70.png);
border-color: black;
}

So add this to your theme's stylesheet:

[css]
.gform_wrapper textarea {
background: white;
}