diff --git a/app/models/site.rb b/app/models/site.rb
index d806863a..bd382131 100644
--- a/app/models/site.rb
+++ b/app/models/site.rb
@@ -460,9 +460,9 @@ class Site < ApplicationRecord
     new_configuration = configuration.dup
 
     if design.no_theme?
-      new_configuration['theme'] = design.gem
-    else
       new_configuration.delete 'theme'
+    else
+      new_configuration['theme'] = design.gem
     end
 
     new_site = Jekyll::Site.new(new_configuration)