Twenty Fourteen is designed around a maximum width of 1320 pixels. You have a couple of different options, depending on exactly what you want to do.
First, you could set a background color or image by going to Dashboard > Appearance > Customize > Colors or Dashboard > Appearance > Customize > Background Image. You could also center the site within a larger browser window by using CSS in your custom CSS plugin or a child theme:
.site {
margin: 0 auto;
}
Another option would be to override the max-width (again using a custom CSS plugin or a child theme):
.site,
.site-header {
max-width: 99999px;
}
However, be aware that the theme is designed around a maximum width of 1320 pixels and if you take the last option, your featured images may look a little stretched out or there may be some other unintended visual issues.
