Ticket #28573: 28573-2.diff

functions.php

@@ -404 +404 @@
 
         if ( get_header_image() ) {
                 $classes[] = 'header-image';
-        } else {
-                $classes[] = 'masthead-fixed';
+        } elseif ( ! in_array( $GLOBALS['pagenow'], array( 'wp-activate.php', 'wp-signup.php' ) ) ) {
+                // Checks if not in Multisite signup and activate pages. 
+                $classes[] = 'masthead-fixed'; 
         }
 
         if ( is_archive() || is_search() || is_home() ) {