Hello Ghoinaulloi37
From what I know, SBB is not liable for any stolen items, provided the locker was functioning properly.
The terms of use for the lockers can be found inside the locker door. The service is based on the Obligationenrecht (Code of obligations), article 253.
https://www.fedlex.admin.ch/eli/cc/27/317_321_377/en#art_253
You can contact SBB in writing if you are unhappy with the situation:
SBB Contact Center.
Postfach
CH-3900 Brig
Furthermore, you should contact your insurance as well.
Kind regards,
Roland