Support `prefers-color-scheme` media query in AMP emails (Dark Mode)

### Description

Gmail's automatic dark mode for emails can be hit or miss, and I think AMP has the potential to improve on it by supporting dark mode media queries like this:

```css
@media (prefers-color-scheme: dark) {
/* dark-mode-only CSS */
}
```

For example, I use images to display the trash icon and plus button in the email below:

| Light Mode | Dark Mode |
|------------|------------|
|  |  |

But these images automatically get inverted on dark mode, making them appear invisible.

I wish I had more control over how dark mode renders my emails.

### Alternatives Considered

An alternative (for my use case, anyway) would be allowing `