Dark mode (initial cut)

[rewbs]

An initial version of darkmode. It mostly looks good but I'm not a darkmode user so haven't explored it in detail. There are most likely more tweaks required. New colours can be added in theme.ts (see references to existing colours for examples of how to use them).

Thanks to @conorluddy for getting this started.

[github-actions[bot]]

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[conorluddy]

My pleasure, @rewbs - will try to contribute more when I'm less busy 🚀