docs: Improve Mermaid diagram SVGs in dark mode

[aefhm]

Description

Relates to #308.

Mermaid diagrams currently are exported to an SVG that is read inline which keeps a white background even in dark mode.

We would like to reduce the contrast and improve the UX for devs using dark mode. Mermaid does not currently support dark mode detecting SVGs through CSS, but does pass themes into exported SVG and also supports overrides.

Themed images is another approach. I encountered some difficulties during the last attempt as inlined SVGs lost some attributes, but we can give it another try. Other approaches are welcome too.