One of the awesome things about MLP:FiM is the really strong and diverse personalities of the characters in the show. It's not surprising that people think up musical themes for each of them.
Well, DJDelta0 wrote a theme for Twilight Sparkle called Methodical. It's supposed to represent the way Twilight might think in class or when she's studying. I think DJDelta0 did an amazing job on this piece—I really like it! Give it a listen in the video, or download it.