For this topic - simply stating it as "Theory" seems inadequate. Though given the diversity of what this could entail maybe titling this in such a non-specific way is what is appropriate. With what has been posted so far, there are some hints. This site's "Intro" post includes a reference to earlier efforts at writing on this topic, and the posts relating to hymns also offers a few clues. Also of consideration is theory as it relates to working on original compositions.
The other aspect if this is that in some ways these posts can offer instructional information, though for being music lessons per se, I am not sure what the best approach would be for trying to do this as an online project if that were to be one of the goals for this site. By way of example, the post relating to instructional Worksheets does provide some materials and the "Chord Pairs" post expands on this a bit more for expressing the challenge this would be.
More to come, though given the diversity of what "theory" can mean, not sure what direction it will take us yet.... Time will tell....
