Button objects aren't classic objects as they are native html form elements. Therefore, they are handled by CSS. You can set an ID in the button properties, and then apply a CSS style to it through your website. I don't know other methods for doing this.
Another solution would be not to use the default button object, but create a sprite and add everything that could make the user believe it is a button (some kind of hoover, a onclick, some visual effects, you could also use the spritefont, ...)