Style your Drupal theme according to the time of the day by adding a "night" or "day" classes to the body.

time_class

/**
* Implements template_preprocess_html().
*/
function YOUR_THEME_preprocess_html(&$vars) {
  // Day / night background theming.
  $hour = date('G');
  $theme = 'day';
  if ($hour <= 6) {
    $theme = 'night';
  }
  elseif ($hour >= 6 && $hour <= 18) {
    $theme = 'day';
  }
  elseif ($hour >= 18) {
    $theme = 'night';
  }
  $vars['classes_array'][] = $theme;
}