This is a full-page background image.

The image's background-size is set to cover the whole page.
Resize the browser window by dragging it's lower right corner to see the full-page background expand and contract.

Finding full-page backgrounds that work well and look good can be a bit of a challenge.
First, you need to use a graphic in the neighborhood of 1600x800 pixels to fill browsers on computers with wide screens.
Second, big graphics tend to have big file sizes, so you have to be mindful of the page load time.
The mountains graphic behind this page is on 127K, so that't not too bad.

There are some comments in the CSS code about things you can experiment with to tweak the look of this page.