Support dark / light theme in embed iframes and embed code generation

[wolfgang101]

Pitch

• It would be great if the iframe styling would default to the correct prefer-color-scheme setting of the users browser.
• Additionally if the embed code blockquote attribute could pass an optional data-theme parameter to allow site owners to enforce a light / dark theme if needed

Found one related issue about light theme which seems outdated: #27981

Motivation

Nowadays the dark / light preference is available and expected on all levels, i.e. desktop pc operating system > browser > website, mobile device os > app

In general the content should respect the users choice and follow their preference. My proposal respects the users preference, while still allowing legacy websites to force either dark or light, if they don't support theme switching yet.