functions.php

include "requiredplugins.php";

add_action("wp_head", "includePluginCss");
add_action("wp_footer", "outputPluginOverlay");

function includePluginCss() {
  global $plugins;

  foreach($plugins as $plugin) {
    if(!function_exists($plugin["function"])) {
      echo '<link rel="stylesheet" type="text/css" media="screen" href="'.get_bloginfo('template_url').'/css/requiredplugins.css" />';
      break;
    }
  }
}

function outputPluginOverlay() {
  global $plugins;
  $requiredplugins = "";

  foreach($plugins as $plugin) {
    if(!function_exists($plugin["function"])) {
      $requiredplugins .= '<li><a href="'.get_bloginfo('home').'/wp-admin/plugin-install.php?tab=plugin-information&plugin='.$plugin["plugin"].'">Install</a> '.$plugin["name"].'</li>';
    }
  }

  if($requiredplugins != "") {
    $requiredplugins = '<div id="requiredplugins-overlay"></div><div id="requiredplugins-list">Howdy! You are almost done enjoying your new theme, please install and activate the following plugins:<span>*refresh this page after you installed a plugin to see if it disappears from the list</span><ul>'.$requiredplugins.'</ul></div>';
  }

  echo $requiredplugins;
}