Looks good, I think there is a great foundation. The Pentatonic Scale feels out of place; but that's probably because I feel that pentatonic is just a mutation of other modes. Chords are really just stacked intervals, it might make sense to move them immediately after the 'harmony' section? The interface is really nice, but I don't think it clearly communicates or reinforces the idea of 'tonic', which is fundamental to understanding music theory. Please keep working on this, there needs to be a good interactive explanation of music theory!

EDIT: You might benefit from filling out the intervals section. Clearly articulate each interval, which are classified as consonant vs dissonant, etc. That would get the user/student thinking about 5ths vs 7ths, etc.