The most likely cause for this is corrupted file format caused from a connection problem or unzipping the theme/plugin zip file.
Downloading and installing the theme/plugin zip file again should resolve the problem.
Make sure when you download the theme/plugin, it remains as zip file on your system, any other file format than zip will show cause this error when installing.
Note: If you are using a Mac with Safari, your zip files will automatically be unzipped. You will need to adjust your Safari Preferences below:
- Open Safari
- Click Preferences
- Under the General tab, uncheck the option Open “safe” files after downloading
- Download the file again.