[Watcher] Webhook password field breaks dark mode when filled

[yuliacech]

When testing https://github.com/elastic/kibana/pull/180331, I noticed that in the Watcher plugin there is a small issue when using the dark mode. To reproduce:

1. The browser needs to have a password saved and configured to automatically fill out the fields
2. Enable the dark mode in the Advanced settings
3. Navigate to Watcher and start creating a threshold alert
4. Add a webhook in the dropdown "Add action"
5. The pre-filled password field is not displayed correctly in the dark mode

Screen recording

https://github.com/elastic/kibana/assets/6585477/af210331-1c42-4d41-8960-11a8b7d4bb1e

Screenshot

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)