Some CSS to change the attributes of the ‘single_add_to_cart_button‘ CSS class (which is used for the button; I found that while using Firebug to inspect the button element).
.single_add_to_cart_button:hover {color:#ccc; background-color:#444;}
Adjust the colors as desired. Put this in the ‘additional CSS’ of the theme customization.
Here’s a place to figure out colors: https://www.w3schools.com/cssref/css_colors.asp .