If you’re getting a 404 for the custom.css, it simply means the file does not exist at that location. So either it was not created or created in the wrong location. Naturally I can’t assess anything without seeing the site and knowing where you placed the file, but the first thing you should do is check that your custom.css is actually in the proper location on your server.
If the file is in the proper location then it’ll work properly. Compare the path in the URL to the file path on your server for starters.
Hope that helps
