As it turns out I missed changing file name in HTML file as it was calling main.css instead of style.css.
<!--build:css css/main.min.css -->
<link rel="stylesheet" href="https://stackoverflow.com/questions/36839828/css/main.css">
<!-- endbuild -->
Changed it to:
<!--build:css css/style.min.css -->
<link rel="stylesheet" href="https://stackoverflow.com/questions/36839828/css/style.css">
<!-- endbuild -->
An honest mistake but I believe someone else can run into something similar 🙂