Don't know what you use to produce output from your Markdown -- I use MkDocs with Material, and added Mermaid support like explained here.
After some trial-and-error configurations I found out that using Cloudflare's CDN, you can include an older version of MermaidJS with another CSS. This way, I was able to render the diagram using the neutral style:
markdown_extensions:
- pymdownx.superfences:
custom_fences:
- name: mermaid
class: mermaid
format: !!python/name:pymdownx.superfences.fence_div_format
extra_css:
- https://cdnjs.cloudflare.com/ajax/libs/mermaid/7.0.9/mermaid.neutral.css
extra_javascript:
- https://cdnjs.cloudflare.com/ajax/libs/mermaid/7.0.9/mermaid.min.js