Place the custom theme in: /sites/all/themes/custom/

Create the module install file, if it does not already exist. For example, in /sites/all/modules/custom/dfs_partner.install, we add the following to enable and set as default our theme partner_demo:

<?php
/**
 * @file
 * Installation routines for DFS_PARTNER feature.
 */

/**
 * Implements hook_install().
 */
function dfs_partner_install() {
  // Enable custom theme on install.
  theme_enable(array('partner_demo'));
  variable_set('theme_default', 'partner_demo');
}