Form Submit button align not working

I have created a simple, one-field form to collect responses to email to me.  I would like to center the "Submit" button.  In the form settings, I clicked "Advanced," then "Button Alignment: Center."  But the button remains aligned to the left side of the page.  How do I fix this?  Thanks so much!

This issue occurs in Chrome and Edge for Windows and Chrome for Android.  Don't have Firefox, so haven't checked that.

www.middle.st – about 2/3 of the way down

(I'm using Bedford, if that helps)

@RevDun Add to Home > Design > Custom CSS

div#block-yui_3_17_2_1_1576786596207_4690 input.button.sqs-system-button.sqs-editable-button {
    margin: 0 auto;
}

Thanks so much – it worked perfectly!