Post Comment Form: Low contrast on dark backgrounds

[scruffian]

Description

The post comments form textarea is hard to read on dark backgrounds

Step-by-step reproduction instructions

1. Add a group block
2. Give it a dark background
3. Add a "Comments" block
4. Notice that the form textarea (and button) are low contrast (is it because they are disabled?)

Screenshots, screen recording, code snippet

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes