docs: added dark mode

[eddie-knight]

Description

This adds a dark mode feature to the site.

Debatably, it currently infers the user's preference from their system and loads that first. If we add this dark mode option, we may want to make it match other project websites... so we'll need to remove that logic to have it default to light mode until the user presses a button.

Schema Changes

Schema Changes Made

• No schema changes
• Layer 1 schema (schemas/layer-1.cue) changes
• Layer 2 schema (schemas/layer-2.cue) changes
• Layer 3 schema (schemas/layer-3.cue) changes
• Layer 4 schema (schemas/layer-4.cue) changes

Schema Change Details

<!-- If applicable, provide a brief summary or example of schema changes -->

Testing

• Unit tests added/updated
• Manual testing performed
• Test data updated (if applicable)

Related Issues

Reviewer Hints

[jpower432]

Nice! Tried this on a few different browsers and the auto-detection works and the toggling works. I do like the auto-detection from a UX perspective.