Colored border of button with GUI skin

I'm working on a level selection grid. I'm not using the Selection Grid since I have heard that it doesn't allow me to make the different elements (buttons in this case) different colors. I just paint it in onGUI eith a few for loops.

I have made a custom skin, and now I need a way to color the buttons border. I See a option for different border sizes, but I can't really See a difference in the game.

I only want to paint like a thin line around the button, not the whole button.