Index: wp-content/themes/twentytwelve/functions.php
===================================================================
--- wp-content/themes/twentytwelve/functions.php	(revision 21372)
+++ wp-content/themes/twentytwelve/functions.php	(working copy)
@@ -112,9 +112,20 @@
 	 * Depends on Theme Options setting.
  	 */
 	$options = $twentytwelve_options->get_theme_options();
-	if ( $options['enable_fonts'] )
-		wp_enqueue_style( 'twentytwelve-fonts', 'http://fonts.googleapis.com/css?family=Open+Sans:400italic,700italic,400,700' );
+	if ( $options['enable_fonts'] ) {
 
+		/**
+		 * From 3.5 forward, protocol-relative URLs are allowed when
+		 * registering/enqueueing scripts and styles.
+		 */
+		if ( version_compare( get_bloginfo( 'version' ), '3.5', '>=' ) ) {
+			wp_enqueue_style( 'twentytwelve-fonts', '//fonts.googleapis.com/css?family=Open+Sans:400italic,700italic,400,700' );
+		} else {
+			$protocol = is_ssl() ? 'https' : 'http';
+			wp_enqueue_style( 'twentytwelve-fonts', "$protocol://fonts.googleapis.com/css?family=Open+Sans:400italic,700italic,400,700" );
+		}
+	}
+	
 	/**
 	 * Load our main CSS file.
 	 */