You nailed the atmosphere down. I love the moody background and the audio, they really go well together. The green box's mechanic is also very creative, and the puzzles are well designed, especially in the later levels. The spike and swap tiles also help keep the later levels fresh and interesting.
I do enjoy the difficulty of the puzzles, but I think the difficulty curve could be a little better. For one, I think the green box should be introduced later in the game. Keeping track of three boxes, especially when one moves with both arrow keys, is already very challenging. Personally, I would have introduced the spikes first and the green box second to help the player ease into the game. I also think that you should make it more obvious which arrow keys control the blue and pink boxes, since it's quite easy to forget, which can be deadly when a box is near a ledge. I would just add a note on the bottom right saying that WASD is for blue and arrow keys is for pink (or vice versa, I forget). Finally, I'm not entirely sure how the game fits the theme. I get that you control multiple boxes, but I don't see the "stronger together" part of it.
Overall though, very well polished game. I had a ton of fun with the puzzles and spent a long time playing them. They're quite challenging, but it makes them all the more satisfying to beat.