If you get an error while uploading or installing a theme, please check the error message first.
Usually these errors are related to the PHP configuration of your hosting. The theme/plug-in manufacturer specifies values such as «memory limit» for the plug-in/theme to be uploaded without problems.
The PHP configuration can be found in the overview of the Control Panel under «Websites» - «Web settings».
A more detailed guide to php configuration can be found here: Customizing the configuration
Info: Note: If you get an error message with Wordpress, we recommend the following PHP changes:
max_file_upload 128M
post_max_size 128M
max_execution_time 300