In Drupal 7 you want to provide a URL and a theme file in the module that can be overridden in a theme, the big gotcha here is the name convention in hook_theme()

mr_custom_pages.info

name = Custom Pages Module
description = Provides routes and pages with custom themes.
core = 7.x

mr_custom_pages.module

<?php

  function mr_custom_pages_menu() {
    $items['page'] = array(
      'page callback' => 'my_page_view',
      'access arguments' => array('access content'),
    );
    return $items;
  }

  function my_page_view() {
  	return theme('custom_page');
  }

  function mr_custom_pages_theme(){
    return array(
        'custom_page' => array( // this is important, the theme layer will look for this name no matter what is defined below
            // file name will be custom-page.tpl.php
            'template' => 'custom-page', // no matter what this is the theme layer will want the file above so let's keep them the same.
        ),
    );
}