I don't know how to change the appearance of the button, but since you haven't gotten any input, I thought I'd suggest an alternative:
change the color of the cell the number is in. Lets say the min is 3 and the max is 20. You would do this by making the normal pattern color of the cell the min/max pattern color (say yellow). Now put conditional formatting over the cell, that says:
CELL VALUE IS BETWEEN 4 AND 19
... and when this condition is true, change the pattern to no color.
Surprisingly it doesn't seem like there is a way to do what you're trying to do with the spin button control. Given that the functionality of the control is pretty basic, I would suggest you 'roll your own' with a couple of command buttons, and incorporate your functionality there.
(By the way, with VB you can make your own user controls, which is fun. I'm not sure if you can use them in VBA userforms though.)