Quite simple. I found it pretty annoying having to lean forward in my chair to hit buttons after finishing one and checking out the cute art. I'm not sure if this is common knowledge (most likely is ;;) but hey, who doesn't love a cheeky guide!!

Everyone knows that left clicking obviously paints so I hope I won't have to add that here..

Right clicking:

right clicking works as a backspace- this doesn't work while in a picross though (will have to manually click on menu then give up) but it works in any other part of the game.

Hitting right click while in the main hub will also bring up the option to exit the game completely

You can also use this instead of hitting X to check spots off!

Scroll Wheel:

The scroll wheel on your mouse acts as an alternative to C which allows you to mark off spots! Simply hold down on the wheel and let go once done :3