I had a lot of fun with this! Appreciated the option to change colors around, that was neat. Reading the comments a lot of people talk about being able to aim while moving, which I also think would be great.
Something I noticed in that area is that if you grab the magnet midair after jumping it really kills your momentum. Seems like the magnet has a lot of weight to it though, so maybe intentional. It didn't bother me, just noting.
I also have seen a lot of people mention that grabbing a key into level fade out seems awkward. I just happened to replay the tutorial, and in that you grab the key and then leave the level yourself. Maybe that contributes to it feeling wrong later on?
All of the puzzles felt really nice and smooth to go through, except for the last one. I managed to solve it like I did in the image below, which I think is super cool but at the time it was a little frustrating. I can't really offer anything to say in terms of 'improving' the puzzle since I'm not much of a puzzle person, and in the end it was actually very satisfying.
Satisfying enough for me to excitedly take a screenshot of it before even making the jump to finish the level. ^^
I was going to leave this comment without mentioning the art, because I had no issues with it. But, I think it's worth saying that it felt perfectly natural. Everything fit in, looked just right and the only time I looked twice at art was the nice character animations.
Only other things for me to mention would be controls. First instinct was that I should be able to pick up the magnet with left click and throw with right click. I got over it and was used to the controls by the end, but maybe having the option to separate Carry / Enter aim mode into two different inputs is worth considering? Or just transferring over Carry / Cancel aim from the gamepad controls? Being able to jump with Up Arrow as well would be nice considering the other arrow keys can be used to move.
Ended up trailing on and on with this, just want to re-iterate that I had a good time with the demo! Thanks for making it.
