Please visit https://help.market.envato.com/hc/en-us/articles/202821510-Theme-is-missing-the-style-css-stylesheet-error there you can find the solution.
As we noticed recently, a plugin named Word Fence causing the issue, please deactivate it or set it to learning mode if you have it. If you don't have the plugin, probably your server returning an unexpected error on AJAX calls, so I need to see it in place. Please grant us temporary wp-admin panel access.
You are having difficulties to update your bundled plugins via automatic updater?
So how can we update?