Your form?  I’m not sure I have ever had a border around a form.  Can you include a screen shot? You can inspect the form and make some changes to the css.

I'm guessing you'll need to add border-radius to the form style name via custom css, something like .frm_style_formidable-style, depending on your style name. That isn't an option in the general form settings.

If you're using a page builder or a block builder, can add styling in the properties panel for the container the form is in?