diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_block.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_block.yml
index 9583d69..0631d53 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_block.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_block.yml
@@ -59,6 +59,14 @@ process:
       - region
       - theme
       - @theme
+    map:
+      left: sidebar_first
+      right: sidebar_second
+      sidebar_first: sidebar_first
+      sidebar_second: sidebar_second
+      help: help
+      header: header
+      footer: footer
   label: title
   'visibility.path.visibility': visibility
   'visibility.path.pages': pages
diff --git a/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/process/d6/BlockRegion.php b/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/process/d6/BlockRegion.php
index 59b3f4a..6d6807a 100644
--- a/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/process/d6/BlockRegion.php
+++ b/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/process/d6/BlockRegion.php
@@ -6,6 +6,7 @@
 
 namespace Drupal\migrate_drupal\Plugin\migrate\Process\d6;
 
+use Drupal\Component\Utility\NestedArray;
 use Drupal\migrate\MigrateExecutable;
 use Drupal\migrate\ProcessPluginBase;
 use Drupal\migrate\Row;
@@ -32,17 +33,9 @@ public function transform($value, MigrateExecutable $migrate_executable, Row $ro
 
     // If the source and destination theme are different, assume these mappings work for most
     // themes (source => destination).
-    $default_region_map = array(
-      'left' => 'sidebar_first',
-      'right' => 'sidebar_second',
-      'sidebar_first' => 'sidebar_first',
-      'sidebar_second' => 'sidebar_second',
-      'help' => 'help',
-      'header' => 'header',
-      'footer' => 'footer',
-    );
-    if (isset($default_region_map[$region])) {
-      return $default_region_map[$region];
+    $new_region = NestedArray::getValue($this->configuration['map'], array($region), $key_exists);
+    if ($key_exists) {
+      return $new_region;
     }
 
     // Oh well, we tried. Put the block in the main content region.
diff --git a/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/process/d6/BlockTheme.php b/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/process/d6/BlockTheme.php
index 9f23703..bad88ef 100644
--- a/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/process/d6/BlockTheme.php
+++ b/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/process/d6/BlockTheme.php
@@ -70,8 +70,9 @@ public function transform($value, MigrateExecutable $migrate_executable, Row $ro
     $d8_default_theme = $this->config->get('default');
     $d8_admin_theme = $this->config->get('admin');
 
-    // D6 theme and D8 theme are the same - sweet!
-    if ((strtolower($theme) == strtolower($d8_default_theme)) || (strtolower($theme) == strtolower($d8_admin_theme))) {
+    // If the source theme exists on the destination, we're good.
+    $themes = \Drupal::service('theme_handler')->listInfo();
+    if (isset($themes[$theme])) {
       return $theme;
     }