Automatically set mermaid themes to match dark / light

[edavidaja]

currently I'm using a reveal presentation with theme: night and adding

%%{init: {"theme": "dark" } }%%

to each diagram. I'm not sure if this works for every diagram type--the mermaid website suggests that theming, for example, state machines has to be done via a different mechanism.

[cscheid]

The problem is that figuring out which themes should be "dark" and which should be "light" isn't obvious.

I do think what that we should have a yaml option to directly configure themes, though, that's a great idea.

[mcanouil]

Closing in favour of #1530