Customizing a button in a NinjaForms form

All of the buttons on the website have round corners, except one that is in a NinjaForms form. By default, the form button has a plain grey style, but within the NF plugin, you can apply an extra class. With the class “button” applied, the button takes on the colours of the theme settings. The only feature that it hasn’t taken on is the rounded corners. I’ve tried creating a new class in CSS but can’t get it to have round corners.