Go to Administration Center > Configuration > Themes and Layout > Manage and Install.
In this page you can install a new theme by:
- Clicking "Install a New Theme" and "From a file:", uploading a file that you've downloaded, (i.e. downloaded from the SMF Theme site) STILL ARCHIVED.
- selecting an existing directory on the server (i.e. a theme you previously uploaded)
- duplicating the default theme