Comprehensive documentation for class references (properties, methods, signals) are accessible in Godot directly. Press F1 and search for Theatre classes, or hold Ctrl and click one of the class in the script to see its documentation page. I'm too lazy to migrate all of that stuff here this site is focused for in-depth tutorials and guides.
Warning
Theatre and this documentation is still under development. The contents of this documentation are subject to breaking changes, and might not fully reflect the current Theatre version.