Quantity Arrows

My website is https://primrose-gar-cl8a.squarespace.com/

I have been looking everywhere and cannot find a solution so maybe its just not possible. 

I want to change the arrows on the quantity box listed on the product page. Instead of up and down, I was hoping to have a - icon on the left of the quantity and a + on the right of the quantity like the picture attached. I can't even find a plugin!