An SMF theme is what defines the look and feel of your forum -- what is sometimes referred to as "skin". Every new installation of SMF comes with the default theme. The administrator is free to install other themes, and to choose which of the installed themes to enable.

Themes are managed in the Themes and Layout section of the administration center -- theme settings and options can be change, new themes can be installed or created as copies of existing themes, and even edited from here. SMF comes with a limit of 256 installed themes, and Themes and Layout administration is designed to deal with fewer than 100 installed themes.

Each theme consists of a collection of templates written in php, plus translatable text strings (in language files), accompanied by images, CSS stylesheets, and other files. Together, these create the HTML, the colors, pictures, and other styling, and the interactive actions users find on the completed page. The SMF software gathers the content to be put in the templates before calling any template files to start writing HTML This separation of content from layout makes it possible to completely rearrange the way content is presented on a page without needing to make any changes to how the software creates the content. The Developing Themes category has more information on creating themes.