How to fix missing stylesheet problems when installing a WordPress theme ?

When trying to install a theme in WordPress, you might sometimes get the following error messages :

The package could not be installed. The theme is missing the style.css stylesheet.
or
ERROR: Stylesheet is missing.
or
The active theme is broken. 

Why does it happen ?

1. This problem is usually happening when you are trying to install an unzipped folder, instead of the .zip file of your theme. You are trying to upload other than ZIP Compressed version ( what is a .zip file ? ). Other compression formats are not supported while uploading a theme or a plugin in wordpress. When installing any plugin or theme in WordPress, the file should be uploaded as .zip file.
2. ( rare ) The .zip file is incomplete or corrupted. In this case you need to download a new version of the .zip file for the theme or plugin that you're trying to install, and upload it again in WordPress.

Note for Apple device users : If you are using an ios device like a Mac, you might need to change your preferences in Safari to stop unzipping the file automatically. More about this here : http://apple.stackexchange.com/questions/961/how-to-stop-safari-from-unzipping-files-after-download

How to solve the issue ?

Download the .zip file of the theme again, make sure you don't unzip it, and re-install the correct zip file of the theme as follow :

• Log in to your WordPress admin area.
• Browse to Appearance > Themes.
• Click on ‘Add New’.
• Click ‘Upload’.
• Browse and select the theme .zip file to ‘Upload’.