diff --git a/core/modules/system/src/Plugin/migrate/destination/d7/ThemeSettings.php b/core/modules/system/src/Plugin/migrate/destination/d7/ThemeSettings.php
index 4c70272..53e37e3 100644
--- a/core/modules/system/src/Plugin/migrate/destination/d7/ThemeSettings.php
+++ b/core/modules/system/src/Plugin/migrate/destination/d7/ThemeSettings.php
@@ -87,6 +87,8 @@ public function getIds() {
    * {@inheritdoc}
    */
   public function fields(MigrationInterface $migration = NULL) {
+    // Theme settings vary by theme, so no specific fields are defined.
+    return array();
   }
 
 }
diff --git a/core/modules/system/src/Plugin/migrate/source/d7/ThemeSettings.php b/core/modules/system/src/Plugin/migrate/source/d7/ThemeSettings.php
index 3f82688..bda861a 100644
--- a/core/modules/system/src/Plugin/migrate/source/d7/ThemeSettings.php
+++ b/core/modules/system/src/Plugin/migrate/source/d7/ThemeSettings.php
@@ -13,10 +13,6 @@
  * )
  */
 class ThemeSettings extends VariableMultiRow {
-  /**
-   * {@inheritdoc}
-   */
-  protected $variables;
 
   /**
    * {@inheritdoc}