wpismypuppet on "Add search box in footer"

You simply add a style!

#searchform {
    width: 88%;
}
#searchform div {
    background: #ffffff;
}

The first style keeps the form from hitting the right edge, and the second one adds a background color of white. There is already a style in place that makes the width 99% (line 1117 in style.css), so you should really find that and alter it instead of creating a new one.