The general aim is to make a fully featured skin that emphasises both content and editing tools, with multiple view modes for showing everything, focusing only on content (winter), or switching to dark/night viewing. One day it will actually do this.
Set it as you would $wgLogo to an appropriate background image. Recommendation is an svg set to 500-750px wide: svg for HiDPI support; the size can really be whatever, but where that will result in a similar behind-content look as with the default cat.
For a background to potentially also work unmodified with other background-colour themes if and when they are later implemented, a transparent single-colour image is recommended, with 20-50% opacity black/white, where the alpha values of the b/w parts are likewise around 10-50/255, as this should allow it to work on most anything that isn't pure white/black.
This skin is being used on one or more Wikimedia projects. This probably means that the skin is stable and works well enough to be used by such high-traffic websites. Look for this skin's name in Wikimedia's CommonSettings.php and InitialiseSettings.php configuration files to see where it's installed. A full list of the skins installed on a particular wiki can be seen on the wiki's Special:Version page.