Recently someone else had a similar issue about implementing Custom CSS.
The Papermod theme does provide for an easy way to add custom CSS.
See the following post as well as the rest of the topic: Papermod Theme: How to add custom CSS?
The Papermod theme repo is located here and its demo over here
If the above theme does not suit your needs have a look at the Hugo Themes Showcase. There is a plethora of themes, some of them provide for CSS overrides and/or Mathjax, of course in every theme the way of doing things is slightly different, due to the preferences and habits of each author.