form imbedding with custom CSS to center submit button even when resizing browser

I'm trying to build a form to imbed onto our site where the submit button is always centered. It has 2 rows and 3 columns. Aligning it to the center simply doesn't do it.  When looking at it on a phone all the field react and turn into one column but the submit button is way to the right. What CSS can I add so it is always in the center?

Thanks