add dark mode toggle

[aqandrew]

requirements

• [ ] respect system UI theme by default
• [ ] store choice in localStorage

the Astro tutorial includes an article for this: https://docs.astro.build/en/tutorial/6-islands/2/

[aqandrew]

https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/light-dark will be helpful for defining theme colors