mermaidjs diagram arrows are too dark in dark mode

[listx]

If you do doctave init and serve the default example.md, the sample diagram's arrows are too dark in dark mode, making it hard to see.

[listx]

I also noticed that reloading the page (after enabling dark mode) makes it the reverse --- the arrows are bright but the nodes are too dark, making the nodes hard to read.

[begleynk]

Thanks for reporting this. Maybe we can tweak the default theme a bit to make these more readable.

We're also going to bump the Mermaid JS version (#17). We may want to do this at the same time.