This is the official documentation. Use the list of pages on the left to navigate.
Theos was initially ‘iphone-framework’, a project created to simplify building code at the command line, primarily for jailbroken iOS devices. It later underwent significant changes and became Theos, a flexible Make-based build system targeting jailbreak software development, but also with complete support for building for various other platforms. Theos runs on, and can build projects for, macOS, iOS, Linux, and Windows.
In 2015, the project stalled as Dustin Howett was unable to devote time to it. Since then, it has been taken up by the community who have added a host of desired features and fixed numerous issues. The legacy version of Theos that used to be located at DHowett/theos is now permanently preserved on the legacy branch.
Documentation for the original Theos is available on the iPhone Dev Wiki and at theos-ref. Please note, however, that not all of said documentation is up-to-date and may not reflect recent changes to Theos.