docs: Adjust registration specifics for ThemeService

[morning4coffe-dev]

GitHub Issue (If applicable): closes unoplatform/uno#19077

PR Type

What kind of change does this PR introduce?

• Documentation content changes

What is the current behavior?

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Docs have been added/updated which fit documentation template. (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Updated the Release Notes
• Associated with an issue (GitHub or internal)

Other information

Internal Issue (If applicable):