Expanding the width?

It’s actually not always simple – as layout changes may affect many pieces of a theme. But you could try changing (or removing) the max width here:

#wrap {
    background: none repeat scroll 0 0 transparent;
    margin: 1em auto 0;
    max-width: 50em;
    padding: 2em 4em;
}

Do be aware that changes will be overwritten when the theme is updated – to avoid that you can use a custom CSS plugin and in that case, you would add:

#wrap {
    max-width: none;
}