diff --git a/mailsystem.theme.inc b/mailsystem.theme.inc
index 8a22b91..d4e6534 100644
--- a/mailsystem.theme.inc
+++ b/mailsystem.theme.inc
@@ -26,6 +26,10 @@ function mailsystem_theme_theme_registry_alter(&$theme_registry) {
     if (isset($themes[$mailsystem_theme])) {
       $theme = clone $themes[$mailsystem_theme];
       if (isset($theme)) {
+        // Swap to the mailsystem theme
+        $old_theme = $theme_key;
+        mailsystem_theme_swap_theme($mailsystem_theme);
+
         // Establish variables for further processing.
         $base_theme = array();
         if (isset($theme->base_themes)) {
@@ -75,8 +79,46 @@ function mailsystem_theme_theme_registry_alter(&$theme_registry) {
             }
           }
         }
+
+        // Swap back to the original theme
+        mailsystem_theme_swap_theme($old_theme);
       }
     }
   }
   $recursion_prevention = FALSE;
 }
+
+/**
+ * Helper to swap themes safely for use by mailsystem_theme_theme_registry_alter().
+ */
+function mailsystem_theme_swap_theme($new_theme) {
+  // Make sure the theme exists
+  $themes = list_themes();
+  if (empty($themes[$new_theme])) {
+    return;
+  }
+
+  // Both theme/theme_key are set to the new theme
+  global $theme, $theme_key;
+  $theme = $theme_key = $new_theme;
+
+  // Create the base_theme_info
+  global $base_theme_info;
+  $base_theme = array();
+  $ancestor = $theme;
+  while ($ancestor && isset($themes[$ancestor]->base_theme)) {
+    $ancestor = $themes[$ancestor]->base_theme;
+    $base_theme[] = $themes[$ancestor];
+  }
+  $base_theme_info = array_reverse($base_theme);
+
+  // Some other theme related globals
+  global $theme_engine, $theme_info, $theme_path;
+  $theme_engine = $themes[$theme]->engine;
+  $theme_info = $themes[$theme];
+  $theme_path = dirname($themes[$theme]->filename);
+
+  // We need to reset the drupal_alter and module_implements statics
+  drupal_static_reset('drupal_alter');
+  drupal_static_reset('module_implements');
+}