Good one, changing the gravity is not something i would have thought of its not hard to control once you get used to and can be quite responsive if slow.
Ill say what others have pointed out, the fireballs come out of nowhere and there is no way to dodge them so passing by most is a roll of the dice with no decision in the matter.
Theme, i think it just about fits, not tremendously but you do use only one control.
Originality, as i said before not something ive seen or thought, at least not to this degree of control over the gravity, the closer i can think of is VVVVVV.