The easiest way that has worked for me is to have your css/js/img files in your webroot - and they should be there because they are meant for pulic viewing. You can simply create a 'module_name' folder in your webroot to house your public files

-apps
--cms(module)
-webroot
--cms(module_public_files)
---css
---js
---img

Just for simplicity and then your URL will be ../cms/css/whatever_file