I noticed it when they introduced Seven.
Every place on the ship before her was fairly well lit and comfortable.
They at first made Seven's cargo bay a very dark and creepy.
Everybody that dare enter, did so with apprehension.
How most eps involving her reminded you that being Borg, still made her dangerous.
The tone became darker.
Janeway even had gone from being optimistic when offered a quick way home, too being pessimistic that it was just another hoax. After season 3, Janeway was less interested in discovering new quasars of the DQ and looking for more direct routes to an energy source.