Styling buttons on CiviCRM public forms
If you're using Drupal + CiviCRM you should check out the "Shoreditch" theme for CiviCRM which changes the CiviCRM UI to use bootstrap based styling.



Note: Shoreditch is in Alpha state and undergoing active development, we are using this in production and aside from a few minor issues it's working very well.

If you're not using Drupal you would want to look at overriding the CSS in your CMS/site theme. You can get the button classes etc using the "Web Inspector" in any major web browser - these are not directly documented at present.