It says the theme is broken or missing a style sheet?
If after installing the theme it says it is broken or the style sheet is missing it's nothing to worry. This is one of the most common errors new users to WordPress experience.
Unzip the zip file that you downloaded from ThemeForest, you should then see another zip file with the themename followed by the word theme e.g. themename-theme.zip. Unzip this file and upload the zip file that is just the theme name e.g. themename.zip.