Caldera Forms uses Bootstrap 3 for its CSS. By default all of the parts of Bootstrap needed for Caldera Forms to look good are loaded. If your theme already loads Bootstrap, or you want to use all custom styles, then you should disable loading Caldera Forms CSS. You can do this from the General Settings.
In the main Caldera Forms admin page, click the General Settings button and set the first three switches — Alert Styles, Form Styles and Grid Structures — to disabled.