Cyan's Theme is written in the key of C Dorian.
According to the Theorytab database, it is the 4th most popular key among Dorian keys and the 33rd most popular among all keys. The C Dorian scale is similar to the C Minor scale except that its 6th note is a half step higher (A). Chord progressions in Dorian have a characteristic sound due to the major quality of the chord built on the 4th scale degree.
See the C Dorian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!