Thanks for contributing to our forum. We do not currently off any functionality like you have described to go along with our Custom Template feature so, if you have already confirmed that the CSS you would like to use is being stripped out by our template editor, it will not be possible to use that specific code in your template.
The template editor on our site should support the use of some CSS so hopefully you will be able to achieve the desired result through some trial and error though.
I hope this helps. If you have any specific questions related to a custom template you are trying to save in your account, please contact our Customer Support team directly from under the "Help" tab within your account and we will do our best to assist you.