How can I center the Next button on mobile?

I have a form in which I've hidden the "back" button, but I want the Next button to still appear. I'd like it centered beneath the Submit button. In the preview mode within Advanced Designer, it looks fine:

But when I return to the builder and test it, it's left-aligned and all wonky. Is there a way to make it centered and responsive?

Thanks!

Please inject the following CSS code to your form: 

.form-pagebreak {

    margin: auto;

    width: 24%;

}

.form-pagebreak-back-container {

    display: none !important;

}

This guide will help you injecting it: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes 

I hope this helps.