Fix opaque background on mermaid diagrams (#21642) (#21652)

Fix opaque background on mermaid diagrams (#21642) (#21652)

Backport #21642

Browsers introduce a opaque background on iframes if the iframe
element's color-scheme does not match the document's color scheme which
in case of a dark theme results in a mismatch and the browser adds a
white background. Avoid this by specifying the same color scheme outside
and inside the iframe.

See https://fvsch.com/transparent-iframes for more info.

My initial attempt was to make the iframe document the same color-scheme
as the parent page (light or dark) but with that, there was a ugly
background flash on load in Chrome because Chrome apparently always
loads iframe in light scheme initially. Firefox still shows a background
flash on load but this is not possible to get rid of and it's certainly
a browser bug.

Before:
<img width="1147" alt="Screen Shot 2022-10-31 at 13 30 55"
src="https://user-images.githubusercontent.com/115237/199017132-9828aace-bdd0-4ede-8118-359e72bcf2fe.png">

After:
<img width="1152" alt="Screen Shot 2022-10-31 at 13 30 36"
src="https://user-images.githubusercontent.com/115237/199017137-989a9e67-3fe0-445f-a191-df5bf290dabf.png">