As a workaround I tried to inject the stylesheets needed via Plugin.
AbstractSkinExtensionPlugin cssplugin =
(AbstractSkinExtensionPlugin) xwiki.getPlugin("ssfx", context);
cssplugin.use(dir+"stylesheet.css",context);
But it failed.
On the other hand,
AbstractSkinExtensionPlugin jsplugin =
(AbstractSkinExtensionPlugin) xwiki.getPlugin("jsfx", context);
jsplugin.use(dir+"ui.multiselect.js",context);
worked as expected.