Automatic colour scheme

[hexylena]

Hey @larsgw, thanks again for writing censordle!

Finally found time to add this since it was bothering me. I (especially) play on mobile, and often outdoors or on the train, and the dark theme is a bit of a contrast struggle for me in heavy daylight.

This PR is split into two commits: one replacing the colours used with CSS variables (so you can be sure they're all the same colours), the second commit adding a 'light' theme, and then using the automatic toggle for dark theme based on the user's system/browser preferences, which is quite well supported.

Image with very minor spoilers for 2023-07-01 (e.g. a country name's prevalence)

Please ignore the yellow, that is from my OS. 

[larsgw]

Thank you very much for the contributions! I just got back from a trip, I'll take a look at these PRs in the coming days.