Added support for light/dark theme toggle in the Playground editor

[filipw]

The playground editor theme is now in based on the current system theme.
There is also a new button to toggle between dark/light themes.

[billti]

Thanks! It would definitely be nice to have, though I think something more wholistic might be preferable. It seems a little odd when the editor changes to dark theme but the rest of the page is still in a light theme.

I did support theming on the katas preview page if you want to see how it's done there. For example, you can open https://microsoft.github.io/qsharp/katas.html and change the system theme to see the whole site change (or you can explicitly open in light mode or dark mode. Would something like that be possible?

[filipw]

Thanks, I wanted to keep the change small, as toggling just Monaco is relatively non-invasive - I didn't know I had the mandate to change the whole thing 😀

I've now pushed a change where the theme management is moved up to the nav component and propagates from there to the editors and the body. I also (obviously) had to introduce a bunch of new CSS.

I might have missed something because I am not sure I know all the nooks and crannies of the playground and what might be shown when, but it seems to work.