[Bug] Thunderbird 91.4.1 Dark Reader not working on reply windows

# Bug Report

## Bug Description
When I try to reply to an email in Thunderbird, there's no icon for Dark Reader and the window is light and not dark.

## Steps To Reproduce
- Open Thunderbird
- Click Reply on an email with a specific style
- See that the email text box is not dark and there's no Dark Reader icon present.

## Expected Behavior
All new windows should have Dark Reader applied, not just new messages.

## Actual Behavior
Reply text boxes in a new window for certain emails are not dark.

## Screenshots

## System Information
Thunderbird 91.4.1

- OS: Windows 10
- Browser:
- Dark Reader Version: 4.9.39

## Additional Context
Can inspect the source of the email if needed.