Adapt Mermaid themes to ZenUML

[MrCoder]

Description

ZenUML has a few built-in themes (they not been ported yet). In the future, it might be a good idea to add mermaid-ish themes to ZenUML.

I will do some research on how this should be implemented. I am an expert in ZenUML but still new to mermaid architecture. Any input and advice are welcome.

Design considerations

ZenUML diagrams are rendered as mostly HTML DOMs so normal CSS is sufficient to customise most of the look and feel.

ZenUML themes