Using the built-in editor in Admin > Customize > Themes might be the best way if you can’t get the CLI to work. The CLI was built because there really wasn’t an ideal way to develop themes for Discourse previously, so I’d like to help you get that worked out if possible.
Are you able to install Discourse locally using any of the methods here: Topics tagged dev-install? You can point the Theme CLI at localhost, which might help avoid any of the network issues you were running into.
Have you tried using a Discourse instance that’s not hosted by nodechef? I wonder if there’s a peculiarity with them specifically (I’m going to try an install there and see if I run into any issues).
Update: Just tried using nodechef and didn’t have any issues with getting the CLI up and running.