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A theme is what defines the look and feel of your forum, and sometimes third-party extensions to it. All installations of SMF should start out with the default theme, although it can be disabled by the administrator.
Themes can be managed in the Themes and Layout section of the administration center. From there, you can install themes, make copies of themes, and change theme settings and options.
A good way to think of themes is as "plugins", which contain a collection of templates, language files, images, stylesheets, and other files. While editing these files can be a bit technical, the layout is separated from the content allowing you to make and distribute or sell your own themes. For information on theme authoring, please see the theme authoring category.
If you are looking to download already made themes, please see the Graphics and Templates board on the community forum.
You are limited, in a standard install, to having 256 themes installed. It is possible to raise this limit, but theme administration has not been designed for themes in the hundreds.