I love how you implemented the theme! Many games have this kind of bonus/drawback mechanic, but this double slider is a great addition and looks like it's so perfectly logical. The art is also very cute. One complaint I have is about controls, I would tie the spellcasting to the mouse button instead of the spacebar. This way the player only has to take care of the keys to control the character, and of the mouse to slide AND cast spell.
Overall Great Work!