How to center buttons and text on stacked images

How do you center text and a button on stacked images? I'm using the Rally template and both are left justified with no option to change under site style. Do I need to use CSS? If so, do you have code suggestion?

You can do this in Design > Site Styles for the Rally template.

If you click on your stacked image it should pull up what you see below. If not, log out and log in again, then try again. 

Under IMAGE BLOCK: STACK

Change Text Alignment to Center. This will center the title, subtitle, and button.