Mermaid not following light / dark themes

[fnchooft]

Bug description

While testing out a very simple mermaid-diagram I noticed that mermaid does not use transparency for background.
As I am using the default plugin I could not get it to work as expected.

Screenshot attached:

Dark-mode:

Light-mode:

How to reproduce

1. Add a mermaid diagram
2. Toggle dark/light mode
3. Observe

Additional context

My book.json in root can be found here: https://github.com/fnchooft/fabcore-organzation/blob/main/book.json