First off, be sure that your WordPress installation, themes and plugins are up to date.

‘Broken Theme’ and ‘Stylesheet Missing’ Issues

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.

First, you can check the installation instructions in the documentation contained in your theme package.

Be sure to do NOT install the whole package.

Usually, the package downloaded on themeforest  is named themeforest-XXXXX-theme-name-description.zip. (e.g : themeforest-4521198-ergo-simple-ergonomic-and-clean-wordpress-theme.zip ).

First, extract this archive pack anywhere on your computer using an archiver (winrar, winzip etc…).

Inside this package you will find the theme folder to install: themename (e.g : ergo), and the “Documentation” folder.

You can upload the theme folder directly through FTP or you can compress it with winrar or another archiver as zip file and install it in the WordPress admin.

You can also check this link.

Why using FTP is better?
The reason why I encourage to install themes via FTP is for the eventual future updates.
If you have to update the theme, you will see that it will be much more convenient to use FTP to replace all the theme files.
This way you won’t have to re-install the theme through WordPress from scratch and re-activate it for each update.

Installation and update with zip fies

In my newest themes such as Flycase it is even more simple. Simply upload the theme zip file from your theme package in the WordPress theme admin panel.

Same goes for the updates. Go to your theme updates panel ( Your Theme -> theme updates ) and simply upload the new theme zip file.