Add support for dark mode

[God-damnit-all]

On https://stylelint.io/demo/ there is a toggle on the header for light mode and dark mode. When in dark mode, the only thing that changes is the color of the header bar.

I tried both a recent version of Chromium (132.0.6834.76) and an older one (118.0.5993.159), both exhibit the same issue.

I made sure my Dark Reader extension was toggled off on the page, though it's worth noting that Dark Reader has a lot of issues trying to turn the page dark.

[Mouvedia]

@jeddy3 if you think it belongs on stylelint/stylelint.io feel free to transfer it again.

[jeddy3]

@God-damnit-all Thanks for raising the issue.

We haven't implemented dark mode in the demo yet.

Please consider contributing if you have time.

Steps:

• explore how the demo can tie into the Docusaurus' theme switcher
• add dark mode to tokens to our demo CSS