The Gramps documentation website uses the MediaWiki content management system to organize and present documentation.
A skin defines a look and feel for the MediaWiki interface. Although the look and feel varies between skins, in general all skins have the same interface components. Each skin can show the same interface component differently, by showing it in different colors or positioning it differently on the page.
To try out the available skins or change to a skin, click on the Preferences link in the upper right while logged in, then click on the Appearance tab. Then click on a radio button in Skin section to select skin and finally the Save button to actually change the look & feel.
You can also preview the available skins before choosing them, by clicking on the Preview link next to each skin.
For more information, see the Skins page on the MediaWiki support site.