A handy dandy plugin that tries to let you download any installed theme as a zip file.
Most themes are licensed under the GPL, which makes it completely legal for you to do whatever you like with them, so long as you don't restrict what other people, in turn, can do with them. This is a type of copyleft license, and it means that you are free to redistribute any GPL-compliant themes any way you like!
Unfortunately, it's often hard to actually do. While it's easy to upload a theme as a zip file, 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 -- including any modifications you've made to the code of it -- as a Zip file that you can then reinstall on any other WordPress site!
P.S. -- If the author of your theme didn't license it on a GPL-compliant license, make sure you're not breaking any licensing terms by what you do with the theme once you've downloaded it!