plus and less button woocommerce

Hi guys!

I’ve the latest version of Woocommerce and WordPress; I’ve installed the plugin WooCommerce Quantity Increment, but I’d like to change the layout of the buttons.

Now: I don’t find the class “plus” and “less”. I’ve tried to add the code “style=”” with some customization, but it doesn’t work.
How I can change it?
Thank you