I like the simple design, the overall idea, and specially how relaxing this game is.
Despite that, even though each puzzle is randomly generated, since gameplay relies on applying the same solving algorithms over and over, this gets boring rather quikly. Solving algorithms can fail if a puzzle has more than a single solution, which instead of making it more challenging, just makes it more frustrating.
For more compelling gameplay, a few things could be twiched, such as:
* Unlocking new difficulty levels instead of having all of them unlocked from start (for a sense of accomplishment);
* Have other shapes instead of just squares (rectangles, yes, but possibly also hexagons or equilateral triangles);
* Negative numbers (or perhaps a level for which by ticking/unticking a number one turns it positive or negative);
* I don't know how mathematically tractable it would be to do so, but have each puzzle have a single solution (this seems to be a NP problem, but I'm not sure);
Anyway, those are a few suggestions off the top of my head. Your game is already quite good as it is. :)