spike: Explore possible dark mode implementations

[alanbsmith]

🔍 Spike / Exploration

We need to compare different approaches to dark mode for Canvas & Canvas Kit.

What questions would be answered once this spike was completed?

1. Do we want a binary light / dark mode or support other color themes? E.g. Light / Soft Dark / Dark
2. How would this affect tenant branding?
3. What are the options for our technical implementation?
4. What are other design systems doing?