Automatically set mermaid themes to match dark / light

[cscheid]

Discussed in #1521

^{Originally posted by edavidaja July 25, 2022}
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.

[mcanouil]

cc @gordonwoodhull

[gordonwoodhull]

Yeah I'm not doing this for 1.7 but I'd like to revisit both Mermaid and Graphviz light and dark themes soon.