problem with skin for a space

Dear xwiki developers, first of all, thanks for XWiki, which is really an awesome tool and one of the most advanced Wikis I know of. I have a problem attaching a skin to a space via the webpreferences of the space. What I did: 1. I created an empty page XWiki.WebpageSkin and attached a new objekt also called "XWiki.WebpageSkin" of class XWiki.XWikiSkins. I removed all contents from the "style" box and entered "THIS IS THE HEADER" into the header box, just to see, if the skin is used or not. 2. I testet the skin by appending ?skin=XWiki.WebpageSkin and to a page and this worked fine. 3. I edited the Main webpreferences (via More Actions/Space Preferences on Main.WebHome) and entered "XWiki.WebpageSkin" in the "skin" box. But this did not take effect. When I did the same for the XWiki preferences (instead of the space preferences), it did work properly, but for all spaces, of course. What did I do wrong? How can I configure different skins for different spaces? Any help would be highly appreciated & thanks a lot in advance & best Lars