The custom.css and all other css files under Themes folder \Website\sitecore modules\Shell\Web Forms for Marketers\Themes gets loaded in web browser on vanilla version of Sitecore 8.1 update 3 but it does not work on my project instance. I believe the developer who worked on it before me has made some changes to pipelines which call these css files to load.

I cannot seem to find anything related to this on internet.

Any suggestions?