Default Mermaid theme makes subgraphs hard to read

[adrhill]

First of all, thanks a lot for your work on Quarto!

I found that when rendering nested Mermaid graphs, the resulting diagrams are close to unreadable, even though the Quarto documentation advertises theming compatibility.
The problem is especially bad when using revealjs.

Revealjs output:

HTML output:

Minimal example:

---
title: "Mermaid test"
format: revealjs
---

## Mermaid test
```{mermaid}
%%| echo: false
flowchart LR

  subgraph B
    direction TB
    C-->D
  end

  A-->B
```

[mcanouil]

I believe this to be a duplicate of:

• Mermaid subgraph background too dark #9178