Marking this as "Implemented" in the next release, even though we treated it as a bug. The feature used to exist and was missing. For completeness, a setting has also been added to set a custom style for the wiki overall.
1. Is there a custom style for this wiki area? If yes, use it. If no,
2. Is there a custom style for the whole wiki? If yes, use it. If no,
3. Does the user have a style preference? If yes, use it. If no,
4. Use the default style for the forums.