It's amazing how faithfully this recreates the old ~2003 Flash game era. The graphics look authentic in a way that's imitated but almost never done right.
This would have been a hell of a game 15 years ago. It's still a decent game now. The puzzles are pretty varied in their difficulty which is good, but the time limit seems unnecessary. Combined with the music, you don't really help the player relax and think and instead you seem to be hurrying them up. Not a good model for a puzzle game.
Other than that there's a lot of things you could have done to make the graphics more impressive. Think about lighting, beveling, texture, etc. Rather than just having text as buttons, put them inside of boxes and give them some level of uniformity. Pick ONE font and stick with it, and unite all of your graphics under one color scheme. The chaotic main menu should have some transparent fill over it to bring the menu items into focus, and what you have makes the green and blue level selection and play screens clash too. The time gauge, mute button, clock, etc. all seem slightly off-center and there's a lot of blank space that seems awkwardly placed.
As far as the game is concerned I think what you have is good. The level design is strong, but some levels really are frustrating. I suppose that's what you want for a puzzle game. Some of the puzzles seem to be out of order in terms of how difficult it is, so perhaps you should have other people tell you how difficult each puzzle is and re-order them so that there's a smoother curve.