Each month, Envato offers a free download. I have downloaded a few WP themes, yet, when I’ve attempted to upload the zip file through my WP admin page, the upload fails because the theme is missing the CSS sheet.
Missing style sheet error when installing the theme
A common issue that can occur with users new to installing WordPress themes is a “Broken theme and/or stylesheets missing” error message being displayed when trying to upload or activate the theme. This error message does not mean that the theme you have purchased is broken, it simply means it has been uploaded incorrectly. Luckily, there is a very easy fix.
The link ThemeSama provided refers to a great & helpful article.
Just a small remark on that article to avoid any confusion: You downloaded the ‘free file of the month’ which downloads the complete package immediately after you press the download for free button. It will not appear in your downloads section.