There is a CSS rule that is limiting the content width:
#page #content {
width: 850px;
}
You can override that by adding this to the end of style.css:
#page #content {
width: 100% !important;
}
If your theme does not allow custom CSS, you should create a Child theme to make your changes. If you do not, all of your changes will be lost if you update your theme.