Changing template for event/contribution forms
Add the following snippet to your theme functions.php replacing page.php with the template you want to use.

Note: you have to be using >= v4.6 of CiviCRM in order to use this.

add_filter('civicrm_basepage_template', 'my_base_template');

function my_base_template($template){
    return 'page.php';
}