Add dark mode to the grid

[Pustur]

Hi, the new dark mode is a very welcome addition, so thanks for that!

However I think that we can make it even better by extending the dark mode to the actual grid as well. As it is right now it's a bit too bright compared to everything else in my opinion.

This PR is a proposal, I'm not sure if I covered all the cases so maybe some more testing is needed.

Here's before:

And after:

What do you think?

[liouh]

The colors you chose look good, however, you're right that it does miss some cases:

• Hover effects are still active after solving.
• An imperfect solve still retains the normal cell colors.

I think it comes down to the fact that using body.dark as additional specificity really isn't a great solution. Two better alternatives:

1. Break out the grid styles into separate CSS files and update the link href in response to theme changes.
2. Apply a body.light rule to the default grid styles (with JS changes to match), so we prevent inadvertent style overrides.

I can also imagine a test.html file that preview all puzzle states at once for testing purposes.

I'm going to close this pull request, but may come back and use the colors you've selected :)