TOS had a variety of tones but for me underneath it's skin , it still felt that it was portraying an overall positive vibe and human outlook . Part of it is the cinematography too .Modern Trek is filmed much more with dark lighting. Like allot of TV is .Even darker than DS9 was shot. That only adds to the dark vibe that makes it even less feel like Star Trek and blends in with other sci-fi. Some would argue it adds a cinematic feel to it. To a degree maybe but I still think its over used. Star Trek 6 had a dark theme but the lighting wasnt too dark. And I don't think the movie suffered for it.
Here comes a somewhat off tangent rant, but cinematography is awful these days. The trend is “natural lighting”, meaning they don’t want you to think there are studio lights but only whatever light sources might be in the environment. Problem is, this makes for a flat, muddy, frequently dark picture. Picard Season 3 was just the nadir; it was shockingly bad. It was often a struggle to see who was doing what.
Back in the day, the director of photography could easily do a darker scene but they still had the sense to use the proper lighting to actually LIGHT the actors. Perfect, artful use of chiaroscuro. Generations is an example of how to do this well. P3 is an example of how NOT do anything visually.