please read and understand the comment above this setting in the configuration:
This is the time setting for the hardcoded root user, not the server. The server has always UTC as internal clock and will transform timestamp to the users profile setting timezone.
root_timezone is the timezone that the user admin - the default superuser - has. His profil is not editable, so you can’t set a timezone outside of the configuration file.
Graylog internally use UTC and display the time always as UTC not with the local time setting of the server.