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The package could not be installed. The theme is missing the style.css stylesheet. Theme install failed...What’s wrong?

You’ve uploaded the entire ZIP file that you downloaded from ThemeForest. You actually need to open that ZIP file first, and inside you can find the theme's ZIP file itself, and reading theme documentation will show you all essential steps. This might be helpfull:

http://support.envato.com/index.php?/Knowledgebase/Article/View/269/0/my-wordpress-theme-isnt-working-what-should-i-do