Update mermaid background

[artberger]

trafficstars

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
• [x] 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

• [x] 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

• [x] 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

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[artberger]

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