Some secret keyboard shortcuts:

Pressing the middle mouse button will change the background color.

Scrolling up the mouse wheel up will darken the background.

The "R" key on the keyboard will reset a level.

"." on the keyboard will skip to the next song.

"Q" and "Escape" will also open the exit level menu.

Wow! You're fast! Thank you very much for playing and supporting the game!

Here's some extra info that might be useful for other players:

While dragging a piece with the left mouse button, pressing the right mouse button will put it right back where it was picked from.

Double clicking a piece that is on the grid will quickly remove it from it, without the player having to drag it.

When many pieces are overlaping outside the grid, right clicking the piece on top will move it to the bottom of the pile, making it easier to sort through them and select the one you want.

There are always four unbeaten levels unlocked, in case you are stuck in one, you can go to the menu and pick another one.

On top of that, you can also skip 5 more levels, in case you're having trouble with a level and want to "mark it for later", you can skip it and it will become temporarily red in the level select menu to signal it for later.

If you beat a skipped level, you will get one of your five skips back.