Missing style sheet error when installing the theme?

If you receive this error, it means you've uploaded wrong zip file to your WordPress dashboard.

You've downloaded All files & documentation, like in screenshot :

Please extract that zip file and inside that folder you can see theme file, documentation, license, and other files. Please upload theme file, that's mostly named as theme-name.zip [eg : juster.zip]

That's the exact file to upload in your : Appearance > Themes

This might be helpful :
Common WordPress Installation Issues