dark and light mode support

This is a placeholder issue for me and others.

My site currently supports light and dark modes. MermaidJS does not support that natively, so I am seeking an alternative.

I was thinking of having two renders per backtick `mermaidjs`.

For example,

```markdown
```mermaid
....
\```
```

Would render to:

```html

@media (prefers-color-scheme: light) {
.mermaid.mermaid-dark { display: none; }
.mermaid.mermaid-light { display: block; }
}
@media (prefers-color-scheme: dark) {
.mermaid.mermaid-dark { display: block; }
.mermaid.mermaid-light { display: none; }
}

```

I'd like to explore this with the plugin.