Installation - How do I install a theme From Online Manual
Although it's very unlikely that installing a theme will destroy your forum, because it's easy enough to revert to the default theme, if required, some members have been known to attempt to install themes that are for different versions of SMF that the theme was designed for. In view of this, we'd always recommend that you backup your forum, before installing a theme.
If you're unsure how to do that, you can read all about that here: Backup
That having been done, we can move-on to the actual installation of themes.
In this page you can install a new theme by: