How to reproduce
- in the XWiki.DefaultSkin, configured as the skin of the wiki
- add a new overridden template, for example print.css but works also for something like layoutExtraVars.vm
- get the URL of that resource, using the xwiki.getSkinFile() mechanism, for example using the following code:
The URL of the actual result displays a "page not found" screen, doesn't return any content.
Note: for regular vms, even if the URL is broken the overwrite is actually taken into account.
However, for the overwrite of a css like print.css since the functioning of the overwrite is based on the URL to the CSS being correctly generated towards the skin file, it doesn't work.
Note: I did the test on a subwiki, not sure if it's relevant or not, but just in case.