hmmm... I don't know about theory, but I've pegged away to try and figure out her tunes by ear and I've found she uses lots of fives and octaves. she also uses majority quarter notes/eighthnotes... her style is pretty circular. she has one theme, then another, then another, and then back to theme one, then to theme two, and so on. She also has a method in many songs of having a "base note" from which she alternates other notes in a riff. such as the beginning to a thousand miles, which goes to and from the middle b, or in white houses, c, c, c, c, g, c, g, c, e, c, e, c, d, c, d, c...
that's all I got.