Visual Design guides

This first batch publishes 2 of the 27 upstream Visual Design guides; the rest are converted and will be released in later batches.

• Dark mode — Implement dark mode support in a way that respects the user’s light/dark theme preference and adapts browser UI (e.g. scrollbars, form controls, etc)
• Ensure text readability with dynamic background colors — Select a readable text color with sufficient contrast on a dynamic background color.