silverwind 57a0a7ef64 Fix opaque background on mermaid diagrams (#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">		2022-10-31 20:57:31 +00:00
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