You can download installed WordPress theme through Cpanel or FTP. But if you don’t have access to either of this then you might be in trouble to get installed theme. Luckily there is a simple plugin named Theme Downloader by which you can download any installed theme as a archived or zip format. While it’s really easy to upload a theme as a zip file, but on the other hand it’s difficult to get the theme back afterwards! This plugin aims to make that a bit easier.
If your server is capable of making Zip archives, and your user account has the edit_themes capability, then this plugin will let you download any of your themes as a Zip file that you can then reinstall on any other of your WordPress site.
First of all get the plugin from above link and install it. After activation go to Appearance->Themes and you will see a new tab to download theme: