If you are going to send a CSS based HTML mail aka Email Templates, chances are you are embedding CSS in <style> tag and end up having a bad experience of not having a properly rendered page.
The issue is that web clients like Gmail etc strip markup tags. Specially <head><script> and <style> for security reasons. The only way is left to make an HTML content with inline CSS.
But.. making inline CSS is pain in neck and it’s not easy specially when are now used to make external CSS files or style block. Thankfully, Mailchimp provides a tool that takes your CSS and HTML and make it online.
Try CSS Inline tool. It’s quite a neat tool and even take care of messy CSS