Changing alignment of button inside widget

Hi guys,

I’ve been setting up a new site and I’m in the process of getting to know WordPress better.

This is the site under construction: http://www.sprachperlenspiel.at
I’m using a theme by GoodLayers (Simple Article) and I’m trying to simply change the submit-button’s alignment in a MailPoet Subscriber widget.

I searched on Google and I also found a thread on the WordPress forums here that deals with a similar issue, but no luck for me so far. I’ve also consulted the official Plugin’s support page to get some more information: http://support.mailpoet.com/knowledgebase/design-style-subscription-form-widget-shortcode/

I just want to center the button. I tried using margin: 0 auto; and text-align: center, tried to change the position etc, but the button won’t budge.

If anyone here has any advice on how to solve this I would greatly appreciate it 🙂