Avatar: The Last Airbender maybe? Some of the DC animated series from the 90s/early 2000s?
The other DC animated series might apply, actually, yeah. I just don't recall them as easily.
Avatar I'd have to say doesn't really work in this situation. It tried its hardest to avoid darker themes in my opinion. There was a gigantic war that was supposed to be happening, and no one died. Ever. There were flamethrower tanks and nobody died. They were never really in danger of dying, either. Like, there was pretend danger, but by the time really serious things started happening, the show had already established that every character had immortal plot armor, even if they were extras (well actually one person died but it was pretty surprising considering, and one of the few instances where a character really didn't have to die). The show did handle some mature themes (maturity itself, healthy and unhealthy relationships, abuse, prejudice, imprisonment, harsh treatment of people, religion, violence), but it avoided from them becoming 'dark'.
No one died
on-screen. This was still a Nickelodeon primetime cartoon, don't forget. But we see plenty of injuries, implied death (watch the Day of Black Sun 1 and 2. Notice how the number of Earth and Water soldiers slowly but steadily decreases until only a handful remain...), and explicit references to death ("Those soldiers are the lucky ones; they came back. Every day the Fire Nation takes lives. People are dying, Aang!").
All things considered, it was still pretty dark and dealt intelligently with mature themes.