How to override the theme in druple while creating the form module

It is not clear what you are asking, are you creating a custom module with a form and want to theme the form?

In your module use this hook to change front theme (for Drupal 7):

function hook_custom_theme() {
  // Allow the user to request a particular theme via a query parameter.
  if (isset($_GET['theme'])) {
    return $_GET['theme'];
  }
}

for Drupal 6 you must use this:

function hook_init() {
  global $custom_theme;
  //here you can change theme
}