Apart from the add-ons manager integration, I’ve recently identified three more implementation parts that need to be tackled for the upcoming lightweight theme feature, which will be based on Personas:
A service that maintains a list of recently used themes and sends a notification when a new theme is selected (the individual windows and the add-ons manager are expected to listen to this). This should live in toolkit, although I’m not quite sure where exactly. A prototype implementation is in bug 511108, feedback welcome.
Front-end code that enables chrome windows to use local as well as remote header and footer images. This should also live in toolkit, but again I’m not sure where exactly. A prototype implementation is in bug 511107, feedback welcome.
A way for web content to install lightweight themes. Details in bug 511771, input welcome.