The zip file from GitHub doesn't include the npm modules required by the extension. You basically have 3 options:
Run npm install in the Theseus directory OR
Delete Theseus and install it using Brackets' Extension Manager instead OR
If you can't use the extension manager for some reason, you can download a working zip file from the Brackets Registry: https://brackets-registry.aboutweb.com/
To test the new sprint 30 version of brackets linux shell: https://github.com/adobe/brackets/releases/tag/0.30.0-8833
I installed thesus (using the gihub master zip file) manually into ~/.Brackets/extensions/user/
but got this error on starting Brackets afterwards (Brackets was not running when I unzip theseus.
I am on ubuntu 12.04 (x64)