I totally agree that this would be an amazing feature. I'm also trying to take notes on NLP syntax and parsing, and don't find the current diagrams sufficient. I can offer that you could write your own CSS rules, but that seems overkill and I don't have the time to do that right now. I will say that since mermaid is open source, you could also add this yourself (or at the very least add it in a fork) if you are so determined (see the docs for this here). One semi-similar feature request on their repo can be found here. I've also personally created a diagram proposal request specifically for this here, so it would be amazing if you could give it some love to build traction.