fix mermaid background rendering

[KaiWaldrant]

Describe your changes

add css rule to fix mermaid background rendering

Issue ticket number and link

Closes #239

Checklist before requesting a review

• [x] I have performed a self-review of my code

• Check the correct box. Does this PR contain:

  • [ ] Breaking changes
  • [ ] New functionality
  • [ ] Major changes
  • [x] Minor changes
  • [x] Bug fixes

• [x] Proposed changes are described in the CHANGELOG.md

• [x] CI Preview succeeds and looks good!

[KaiWaldrant]

@rcannood is this what you want or do you also want a difference in light and dark mode?

[github-actions[bot]]

[rcannood]

Hmmm... Would light grey also look OK?