In general, the compatibility Firefox themes (light or complete) have nothing to do with multi-process. The fact is the old Firefox SDK (XUL) is deprecated and any new extension for Firefox 57 and later has to be done over WebExtensions. As I understand this should mean changing/replacing some files that install/load/describe the resources in the theme. I am yet awaiting for details on what to change from mozilla. However, legacy addons may still be used within Firefox 57 and 58(?) by setting the key extensions.legacy.enabled 'true' in about:config. There is a separate key, extensios.allow-non-mpc-extensions, that could allow non-multiprocess addons, but it has nothing to do with legacy themes. That said, yes, I am behind in updating for the new photon structure for the interface. I was hoping to have a pre-release in the development site today.