Have a rich text button span the width of mobile Device

On legacy forms, when using a mobile device there were some ways to make a rich text button span the width of the mobile device. You could use:

class='Vibrant' or in the styling you could use width: 100%.

Neither of these tricks work on the new forms (on mobile). Does anyone know of  way to make a rich text button span the width of a mobile device on new forms?