Create a new method pdf_stylesheet_pack_tag that actually include CSS file

If running in developpement (ie. `Webpacker.dev_server.running?`), use webpacker to actually render/serve the CSS files.

Source: https://stackoverflow.com/a/60541688

app/helpers/application_helper.rb

@@ -48,4 +48,13 @@ module ApplicationHelper
     classes << "off-canvas" unless @hide_menu
     classes << @shopfront_layout
   end
+
+  def pdf_stylesheet_pack_tag(source)
+    if running_in_development?
+      options = { media: "all", host: "#{Webpacker.dev_server.host}:#{Webpacker.dev_server.port}" }
+      stylesheet_pack_tag(source, **options)
+    else
+      wicked_pdf_stylesheet_pack_tag(source)
+    end
+  end
 end

app/views/spree/admin/orders/invoice.html.haml

@@ -1,4 +1,4 @@
-= wicked_pdf_stylesheet_pack_tag "mail"
+= pdf_stylesheet_pack_tag "mail"
 
 %table{:width => "100%"}
   %tbody

app/views/spree/admin/orders/invoice2.html.haml

@@ -1,4 +1,4 @@
-= wicked_pdf_stylesheet_pack_tag "mail"
+= pdf_stylesheet_pack_tag "mail"
 
 %table{:width => "100%"}
   %tbody