Docs: mention dark colour scheme in design docs

[simison]

Description

Adds a section about dark colour scheme in design docs.

It's easy enough to forget to test with dark backgrounds, just like folks forget to test mobile. Would be great to remind folks to test.

Example what happens when you forget to test: https://github.com/Automattic/wp-calypso/issues/30198

How has this been tested?

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Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[gziolo]

I updated the link to reference GitHub directly. Starting the url with the top folder allows us to do that. This will also work on Gutenberg handbook, it is going to be properly updated by set of regular expression applied behind the scenes :)

Saying that, I'm coming to the conclusion that I need to document all those guidelines which emerged when dealing with links.

[gziolo]

Thanks for this addition, looks good 👍