Dark mode

[microbit-matt-hillsdon]

Suggested by @microbit-katie and it's also come up on support:

• https://support.microbit.org/a/tickets/54534 (private)
• https://support.microbit.org/a/tickets/53302 (private)

Our UI framework Chakra UI has some support for this, but our various customisations also certainly break it.
We'd need an alternative theme for the editor too.
What would we do with the gradient?

[microbit-mark]

The colour scheme also came up in user research a couple of times.

A way to edit the colour scheme and/or have a high contrast mode would be appreciated.

[microbit-matt-hillsdon]

High contrast is tracked separately under the accessibility review.