Dark mode: Mermaid diagrams render incorrectly

Summary

Mermaid diagrams load incorrectly in dark mode (beta)

Steps to reproduce

1. Turn on Dark Mode (beta)
2. Look at an issue with a mermaid diagram, e.g. #211839 (closed)

What is the current bug behavior?

The diagram loads black text on a black background.

What is the expected correct behavior?

Ideally, the diagram would respect dark mode and render accordingly. However, there's a boring solution here where we can give mermaid diagrams an explicit white background.

Relevant logs and/or screenshots

Current	Boring	Ideal?

Possible fixes

Dropping this in for a quick fix works. We'd need to put it in properly though:

svg.mermaid {
  background: white;
  padding: 1em;
  border-radius: .3em;
}