If you want a skin just for a specific forum, there is an option when creating the forum to choose what style to force to be used (Option is Custom Style for this forum then check yes on the next option)
As for a banner, since you're using a separate skin, it'd be easy to just add in a banner to the top (right below the navbar set) or use the forum intro hack thats around to make it add one automatically.
Hope this helps
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