Mermaid should respect RevealJS' theme

[pinage404]

Current Behavior :

RevealJS' default theme is black which is a dark theme

Mermaid's default theme is default which is a light theme

The result is unreadable

Expected Behavior :

RevealJS' default theme is black which is a dark theme

Mermaid's default theme should be black

The result is unreadable

Current workaround :

Manually set Mermaid's theme with each graph

%%{init: {'theme':'dark'}}%%
graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;

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Probably related to #1033