Add dark mode support for visualizer canvas

[Jamesgt]

My OS is set to use dark mode and Monaco editor has dark background, to see the logs the devtools is also in dark mode.
The only part not respecting the preferred setting is the threejs canvas.

[sgenoud]

Thanks! I took the opportunity to improve the dark mode a bit in other places!

[Jamesgt]

I really appreciate!

But now the ErrorOverlay has light bg and lightgrey text, so that should be aligned also.
I don't see it set in the code, so maybe the styled-components lib is adding that default.