Update mermaid background

[artberger]

Changes

In sequence diagrams, if you add numbering, the background circle was too dark to read the numbers in light mode. See screenshots below.

Tests

• Automated tests have been added
• This PR does not require tests

Changelog

• Entry has been added to changelog
• This PR does not make a user-facing change

I am not sure whether this needs a changelog entry or where to do that.

Documentation

• Docs have been updated --> I updated the sample site to add numbering to the sample mermaid sequence diagram
• This change does not need a documentation update

Dark mode

• The UI has been tested both in dark and light mode
• This PR does not change the UI

Light mode before

Dark mode before

Light mode now

Dark mode now

[netlify]

✅ Deploy Preview for hugo-lotusdocs ready!

Name	Link
🔨 Latest commit	15bb49d
🔍 Latest deploy log	https://app.netlify.com/projects/hugo-lotusdocs/deploys/683ddb5c68300e00073b77f4
😎 Deploy Preview	https://deploy-preview-237--hugo-lotusdocs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[artberger]

Hi @colinwilson, I noticed a small issue with the background color in the number of mermaid diagrams, so I opened this small PR.