How can I center all label, input fields and submit button

The first thing that you need to do is to set the label alignment of the fields in your form to "Top".

After that, inject the following CSS codes to your form to make the fields and the submit button in your form centered.

.form-label, .form-input-wide, .form-buttons-wrapper {

    text-align: center !important;

    width: 100% !important;

    max-width: none !important;

}

How-to-Inject-Custom-CSS-Codes