Generally you want to avoid using any of the form-control objects as much as possible, in my opinion at least. There are cases where you have to use them and cases where they work very well actually, but overall they just create headaches, mostly because...
Try using sprites/9patch + "mouse - on object clicked" instead for your button needs. If you absolutely have to use the form-controls, you can only style them with limited CSS, see construct.net/en/make-games/manuals/construct-3/plugin-reference/button
A last ditch option is to place a sprite of how you want the button to look like behind the button object, and set the button object to be at 0.000001% opacity. (Just 0% will not work)