Theme the login page for Maintenance mode
If you want the login page to look different when you are in maintenance mode you theme it by checking the maintenance mode variable like this:

if (variable_get('maintenance_mode', 0)) {
  // Do something because it is maintenance mode.
}


For example, in your page template you could do this:

/**
 * Preprocess variables for the page template.
 */
function THEME_NAME_preprocess_page(&amp;$variables) {
  if (variable_get('maintenance_mode', 0)) {
    // Add a template suggestion for pages that are not the maintenance
    // page, while the site is in maintenance mode.
    $variables['theme_hook_suggestion'][] = 'page__inmaintenancemode';
  }
}


Then you could make a page--inmaintenancemode.tpl.php file, that could be used to theme any pages that are visible while in maintenance mode.
Be aware though that this will then affect all pages you see when you are logged in during maintenance mode, so you might want to change the line that check for maintenance mode to:

if (variable_get('maintenance_mode', 0) &amp;&amp; user_is_logged_in()) {


You can do the same check in hook_form_alter() or in other template preprocessors, or anywhere else you are modifying content.