How to make simple hover effect to buttons?

Exactly what do you mean?
Do you want a shadow? Color change? You have to be specific.

The theory is the same for all of them:

-User mouse pointer is over an Active object (your button)
+(trigger whatever you want to happen)

e.g.

-User mouse pointer is over Button object
+Change objects Effect to invert

-User mouse is not over button object (right click and select negate on an event to make it negative)
+Change object Effect to none.

That would make it invert colors. You can change it's animation to something else if you have drawn something else, or create a shadow object and move it to behind the button if you want shadows.