using another default skin for a space does not work

Using another skin for a specific space (as for the rest of the xwiki) does not work:
skins configured in the web preferences of a space seem to be ignored.

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.