Dark mode for mermaid diagram is not readable

See code: https://github.com/mieweb/opensource-server/blob/5f56b8d45b3c7e3dfddbbd1d247c5cdd7b3aecda/README.md?plain=1#L46

Note the contrast in darkmode is not readable:
<img width="846" height="504" alt="Image" src="https://github.com/user-attachments/assets/670e1f7d-6f36-42d5-b19f-d82f153a1d05" />

