I'm having problems understanding how to override the theme functions.

I want the $mission statement on every page.

At the moment it only shows on the frontpage.

The code for this starts on line 1781 of includes/theme.inc

Code:
 
// Set mission when viewing the frontpage.
  if (drupal_is_front_page()) {
    $mission = filter_xss_admin(theme_get_setting('mission'));
  }
And it is to be found within the function

Code:
function template_preprocess_page(&$variables) { 


}
Solving my problem is quite easy - I just cross out a couple of lines, like so:

Code:
// Set mission when viewing the frontpage.
 // if (drupal_is_front_page()) {
    $mission = filter_xss_admin(theme_get_setting('mission'));
//  }
It works fine but obviously I want this in the template.php and I'm not sure how to do it. I know there are three different types of override - theme, themeengine and theme name, but playing around with these isn't getting me anywhere.