Adventure Time is written in the key of B♭ Dorian.
According to the Theorytab database, it is the least popular key among Dorian keys and the 49th most popular among all keys. The B♭ Dorian scale is similar to the B♭ Minor scale except that its 6th note is a half step higher (G). Chord progressions in Dorian have a characteristic sound due to the major quality of the chord built on the 4th scale degree.
See the B♭ Dorian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!