installation: requirement addition for six library

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
arxiv: docs entry addition

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
docs: css theme overrides

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>