Optimized control of background images

It is now possible to have the default setting to enable background
images (via theme > enable_background_images) but have a domain turn it
off again as they want to use gradients or specific images.
Upgrading any installation that already has any background_image enabled
will cause enable_background_images to turn on to let it continue to
work.
Including support code that the new install system will utilize, should
not affect existing systems.

themes/enhanced/template.php

@@ -1424,7 +1424,7 @@ if (strlen($_SESSION['message']) > 0) {
 
 	<?php
 	// check for background image
-	if (isset($_SESSION['theme']['background_image'])) {
+	if (isset($_SESSION['theme']['enable_background_images']['boolean']) and $_SESSION['theme']['enable_background_images']['boolean'] == 'true') {
 		// background image is enabled
 		$image_extensions = array('jpg','jpeg','png','gif');