subscribe-by-email token textarea uses white background in dark theme

[alexma233]

When the comment form is displayed in dark theme, the token input field in the "Subscribe by Email" flow still renders with a white background.

Steps to reproduce:

1. Open the comment form in dark mode
2. Click "Subscribe by Email"
3. Enter an email and proceed to the token step
4. Observe the token textarea ("Copy and paste the token from the email")

Actual behavior:

• The token textarea has a white background in dark theme

Expected behavior:

• The token textarea should follow the dark theme styling, consistent with the rest of the form inputs

Notes:

• The token field is rendered with TextareaAutosize
• Its stylesheet defines border/padding/focus styles, but no dark-theme-specific background/text colors
• The form already has comment-form__subscribe-by-email_theme_dark, but only the error state seems to have dark-theme overrides