XWIKI-11304: Cannot use, in a subwiki, a skin based on Flamingo stored in the main wiki.

(cherry picked from commit e239694e9b94ced61c368ec16ef0963fa0c44bf8)
XWIKI-11302: Cannot use, in a subwiki, a skin stored in the main wiki and a color theme stored in the subwiki.

(cherry picked from commit 3e83c7c24cac83e7f1f1d79b73b162480d90e8e9)