Index: system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system.module,v
retrieving revision 1.120
diff -u -r1.120 system.module
--- system.module	29 Dec 2003 19:49:11 -0000	1.120
+++ system.module	2 Jan 2004 06:34:03 -0000
@@ -206,6 +206,11 @@
     db_query("UPDATE {system} SET status = '0' WHERE type = '%s'", $edit["type"]);
     foreach ($edit["status"] as $filename => $status) {
       db_query("UPDATE {system} SET status = %d, throttle = %d WHERE filename = '$filename'", $status, $edit["throttle"]["$filename"]);
+      if ($status == 0) {
+        /* Ensure no users are left with themes that are disabled. */
+        $name = substr(basename($filename), 0, strrpos(basename($filename), "."));
+        db_query("UPDATE {users} SET theme = '' WHERE theme = '%s'", $name);
+      }
     }
     if ($edit["type"] == "theme") {
       variable_set("theme_default", $edit["theme_default"]);