add theme

themes/butterfly/layout/includes/page/404.pug

@@ -0,0 +1,8 @@
+- var top_img_404 = theme.error_404.background || theme.default_top_img
+
+.error-content
+  .error-img
+    img(src=url_for(top_img_404) alt='Page not found')
+  .error-info
+    h1.error_title= '404'
+    .error_subtitle= theme.error_404.subtitle || _p('error404')