The code is currently hardcoding the location of the ssx on file:
// There can be some calls to URLs inside the SSX content. For example:
// background-image: url("$xwiki.getSkinFile('icons/silk/folder_add.png')");
// In order for these URLs to be resolved correctly they need to be relative to where the CSS file is located
// on disk. Thus we adjust the CSS path by 3 levels since we're locating the SSX files in "ssx/<space>/<page>".