Desktop,Mobile: Resolves #3201: Render mermaid diagrams in dark mode when Joplin is in dark mode

[personalizedrefrigerator]

Summary

Renders mermaid diagrams in dark mode when Joplin is in dark mode.

Resolves #3201.

Testing

Both mobile and desktop:

1. Create a note with the following content:

# Summary

Renders mermaid diagrams in dark mode when Joplin uses a dark theme.


## Testing

Mindmap:
```mermaid
mindmap
	root((Dark mode rendering))
		Note viewer
			Desktop
				Auto-switch themes works ✅
				Light mode ✅
				Dark mode  ✅
				Editing the diagram causes re-rendering ✅
			Mobile
				Android
					Dark mode ✅
					Light mode ✅
					Auto-switch themes works ✅
				iOS
					Not tested ⛔
		Server/publish
			Dark mode not supported -- as before
			Now has wrong bg color ⛔
		RTE
			Visible ✅
			Auto-switch themes ⛔	
```

Flowchart:
```mermaid
flowchart LR
	a["Test"]
	b["Testing"]
	c["???"]
	d["Test 2"]
	a --> b
	b --> a
	a -.test.-> d
	d --> c
	b --> c
	c --> a
```

gitGraph:

```mermaid
gitGraph
	commit
	commit
	commit
	branch foo
	checkout foo
	commit
	checkout main
	commit
	merge foo
```

mermaid from a `div`:
<div class="mermaid">
gitGraph
	commit
</div>

2. Enable "auto-switch theme" 3. Switch the system theme to dark mode 4. Open the note in the note viewer 5. Switch the system theme to light mode 6. Open a different note, then open the original note in the viewer

Just desktop:
7. Switch the theme to "Dark"
8. Open the note in the rich text editor
9. Switch the theme to "Light"
10. Open the note in the rich text editor
11. Publish the note

12. Open the note on Joplin Server

This has been tested on Android 12 and Ubuntu 23.10.

Known issues

• While the theme does switch in the rich text editor, it does not switch automatically with the system theme. It is necessary to switch to a different note and back for the theme to change.

Screenshots

[tessus]

Nice. LGTM.