CSS is short for Cascading Style Sheet. It is used to define the way that each wiki on Wiki Spot looks.
Wiki Administrators can define the CSS for their site by uploading files to the Wiki Settings/CSS page on their wiki. See Customizing wikis.
Wiki Administrators can find predesigned CSS themes here on Wiki Spot.
Users can add their own styles to apply to the site by providing a link to a CSS file on the User Settings page.