How to invoke a template to format custom variable output from my module? (not pre-existing node)

I am writing a module that will want to display a custom page with some content that varies based on the results of the action the module took. Basically I want to be able to insert my 4-5 variables into specific places in a pre-existing layout. But I don't want to hard-code it in the module. If this was just modifying the display of an existing node I could do it, but I am not sure how to attach a custom template to my output.

At the moment(testing) I am just outputting the variables in a list which gets embedded in the body of a page with all the standard surrounding items (sidebars, header/footer regions and blocks). This is great as far as it goes, I just need more control over that $content area.

At the moment this is how I am doing what I mentioned:

  $list[] = t('The source was @number.', array('@number' => $source));
  $list[] = t('The work type value was @number.', array('@number' => $wrktype));

  $render_array['example_return_arguments'] = array(
    '#theme' => 'item_list',  // The theme function to apply to the #items
    '#items' => $list,  // The list itself.
    '#title' => t('Return Status Information'),
  );

  return $render_array;
}

Is there a way to expand/modify this to call a user defined theme with my variable list? I thought I had seen something that did that once but I can't find it again :-(

Any pointers or help appreciated. And just to state what may be obvious based on the group, this is for Drupal 7.

Thanks