Update HIG theming

[zeebok]

Problem

Currently both Coding and Contributor guides support dark mode and have accent color usage but the HIG does not have either.

Proposal

Add a dark mode. In both other guides, there is a toggle for system preference, light, and dark; this should be included as well. Might be worth considering adding an accent color instead of just being a greyscale page as it is now.

Prior Art (Optional)

No response

[danirabbit]

Ah okay so the reason we don't have dark mode turned on is that it breaks a bunch of the screenshots 😬