Is there a way to center the submit button?

To center the submit button, add the following code to your template.css file or theme.css file. In Build 730 and higher, you will find them at: 

/joomlaRoot/templates/*TEMPLATE-NAME*/template.css

/media/breezingforms/themes/*THEME-NAME*/theme.css

Add this code:

.bfQuickMode .bfSubmitButton
{
     float: none;
     margin-left: 45%;
}

or try this one:

.bfQuickMode .bfSubmitButton
{
margin: 0 auto;
}