Update mermaid.js diagram styling in docs

[discdiver]

Updates styling for consistency and visibility in light and dark mode. Simplifies code for the diagrams.

Checklist

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[zzstoatzz]

this is just a nit, but can we keep some of the color while remaining compat with light and dark mode? I think a bit of color looks nice

[discdiver]

Cool. Agree it would be nice to have some color to liven it up, but think we need to use it consistently and make sure it is visible. I'll merge this PR now as I think it's an improvement over the current state and open something to add some color.