The styling of these elements is defined in the CSS of the theme. In the theme you’re using, h1 is white (because it’s used for the blog title, and the blog title is set against a colored background) and with a huge top padding; h2 is centered.
But h tags have a meaning for search engines and screen readers, so setting some text as an h1 or h2 etc should only be used if that text is actually a heading, not in order to change the size of the text or whatever. To change the styling of the text, you need to use some coding in the HTML editor. See here: http://wpbtips.wordpress.com/2009/07/11/formatting-text-pt-5/