Support dark mode in Editor and sync the theme with the app

[kv9898]

This is especially problematic when the quarto page is in dark mode, which leads to the editor code turning red but the background remaining in white.

[kv9898]

Transferred from quarto-ext/shinylive#63

[georgestagg]

Just as a quick note to myself, the CSS export mechanism mentioned in this post, available in newer versions of Quarto, should give us access to theme values.

The upcoming brand.yml work may also be a good route to the required information.