[PAGE ISSUE]: 'Flutter documentation'

### Page URL

https://docs.flutter.dev/

### Page source

https://github.com/flutter/website/tree/main/src/index.md

### Describe the problem

Add dark mode.

### Expected fix

A button to switch to darkmode.

### Additional context

Hard to read code snippets for color contrast in some screens. And too bright to read docs at night.