Mermaid support improvements 25Q1

[chalin]

trafficstars

Following through from:

• #1961

I have some new insights based on work I've done for the OTel website. Will upstream changes from there when I have a moment.

References:

• https://mermaid.js.org/config/theming.html. As is noted in that page, only base is customizable.
• https://mermaid.js.org/config/schema-docs/config.html#theme and themeVariables
• https://github.com/mermaid-js/mermaid/blob/develop/packages/mermaid/src/themes/theme-base.js and all other theme in that directory.

[sftim]

Improvements here will help Kubernetes a lot; I've upvoted, but I do think this will be really useful for many projects.