My site is not the size of the browser

This is a great theme I really like it. The only issue I have is that the height of the site isn't the full height of the browser, please see the screenshot image I've attached. It's like this when there isn't a lot of content on a page but when a page has a lot of content it appears as it should. It does this in all browsers and mobile devices. How can I set it to be the depth of the entire browser page?

Thanks