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.
Special logo rendering, allowing for custom logos for Timeless specifically (such as a square logo version without the wordmrk in order to avoid duplication with the header). Also provides slightly better HiDPI support than using $wgLogoHD, as that only works for 135x135px logos, and scales all high-res logos down to this size.
Can be used to point Timeless at a File uploaded onwiki (remember to protect the file if using this) simply by providing the file name, or you can specify the logo path details manually using an array.
To reuse a $wgLogo and $wgLogoHD already set for a nominally 160x160px logo, but where the HD versions also come out the right size:
Note that uploaded logos work best as svgs or as 2x- or higher-sized rasters, and will be scaled down for each target resolution. Obviously this approach requires file uploads and thumbnailing to be enabled.
Recommended nominal sizes probably around 135px to 165px.
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.