The plugin has a CSS file but any edits there would probably be overwritten if the plugin gets updated. With that being said, your theme might have a place for you to add custom CSS.
In my case, in the WordPress menu, I clicked on Appearance, then right below it, Edit CSS. The class below is what I added to make my button’s background color black.
.wp-polls .Buttons {
background-color: #000000;
}