amp-facebook-comments doesn't support data-colorscheme="dark"

This is for tracking.

## What's the issue?

For the `amp-facebook-comments` component, **data-colorscheme=dark** doesn't work.

## How do we reproduce the issue?

[Link to AMP Playground](https://playground.amp.dev/?format=websites&_gl=1*grm05v*_ga*YW1wLTh4V2M4ZUdRSjVvVEZJcllRUFA3YkE.#share=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)

To reproduce, simply render a component and set its attribute data-colorscheme="dark".

## What browsers are affected?

All browsers

## Which AMP version is affected?

012007302351001 (latest)