First, make sure you realize that different computers will display web pages at different sizes, so the white space on the sides won't be the same for everyone. Here is an example that will make the wrapper for the Vigilance theme wider. I set the header image to repeat, but you can change out the url() value to an image that is 1220 pixels wide if you'd like to use a different header image.
#wrapper {
width: 1220px;
}
#content {
width: 900px;
}
#header h1 {
background: url('http://mattemrichphoto.files.wordpress.com/2012/02/kc0077vigilance.jpg') center repeat-x;
height: 180px;
width: 100%;
}
Note that the example above will manually reset the header image and if you do it that way, you will be overriding the random header image setting on the Appearance → Header page.
If you wanted to keep the random headers and just center and repeat the image, do this instead;
#wrapper {
width: 1220px;
}
#content {
width: 900px;
}
#header #title {
background-position: center;
background-repeat: repeat-x;
height: 180px;
width: 100%;
}