The theme is set to use white text on green buttons (desktop and mobile), but the form isn’t using white text on the green buttons when under ~478px browser window width. The form isn’t respecting the theme’s mobile settings.
Hey! it seems you may have accidentally set a style on your mobile viewport that you didn’t mean to. You can check it out by toggling the mobile viewport in the top-right corner of your canvas and checking out the styling there. If you want to reset the mobile style back to desktop, you can click the blue tag next to the style and click “Reset to Desktop”