In Sitefinity themes are part of the website templates. You can set custom colors, images, and styles for the different layout templates. The following is a simple scheme of a theme’s folder content:
NOTE: Folder Global is mandatory. If the folder is missing it is not possible to register or load the theme.
NOTE: You can place your theme files in the root folder (~/App_Themes/ThemeName) as it is in ASP.NET convention, but all of your CSS files will be loaded no matter in which folder they are placed (Global, Styles, CSS etc.)