Most themes simply need to be uploaded to the theme folder on your Moodle installation. However, you should read any accompanying text included in the theme you are uploading, usually available as a "readme.txt" file.
Once the theme is uploaded (you can use an FTP client to do this, such as FileZilla) you should see it in the available themes (Site Administration>Appearance>Themes>Theme Selector) or simply type in your domain name/theme and you'll get to this location.
Select the Choose button for the theme you want and it should be installed across your site.
Once the theme is uploaded (you can use an FTP client to do this, such as FileZilla) you should see it in the available themes (Site Administration>Appearance>Themes>Theme Selector) or simply type in your domain name/theme and you'll get to this location.
Select the Choose button for the theme you want and it should be installed across your site.