Did you try a manual install?
1. In your Themes folder, create a new folder and name it the exact name as the theme you want to install. Example: inferno_20
2. Upload the theme zip file to the The folder that you just created using your cPanel.
3. Unpack the theme zip file.
4. Go to Admin> Configuration> Themes and Layout.
5. Where it states "From a directory on the server" type in the exact theme name at the end of the path that is already there.Example: