Revert "Use dark theme by default (#1546)" (#1594)

This reverts commit ab0b9e1.

Author: barryvdh

config/debugbar.php

@@ -308,7 +308,7 @@
      | Switches between light and dark theme. If set to auto it will respect system preferences
      | Possible values: auto, light, dark
      */
-    'theme' => env('DEBUGBAR_THEME', 'dark'),
+    'theme' => env('DEBUGBAR_THEME', 'auto'),
 
     /*
      |--------------------------------------------------------------------------