I had trouble installing a theme, finally figured out the problem, and realized that the troubleshooting instructions could have been better.
TL;DR Add this sentence to the theme’s Support tab and the “missing style.css” page:
A frequent cause of this problem is that the full zip file contains a license file, documentation, and other files along with the “installable Wordpress .zip” file that you need to upload. Luckily, there is a very easy fix.
Here’s how to reproduce:
I downloaded the (free) theme from https://themeforest.net/item/ecobox-responsive-wordpress-theme/8160756
I tried to upload/install, but got a cryptic “Are you sure you want to do this?” message. Huh?
I ultimately found the theme’s support tab/page:
It offered this advice:
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.
That led me to the “Theme is missing the style.css” page at: https://help.market.envato.com/hc/en-us/articles/202821510 This page had the right answer, but… I didn’t think it applied to me
I didn’t originally believe this page applied to me because I had directly downloaded the file from Envato (I figured, how could it be wrong?). Adding the sentence above (“A frequent cause of this problem…”) would have tipped me off that the message does apply to me.
NB: The sentence could be added to both the theme’s support tab, as well as the “Theme is missing the style.css” page.