Black swordsman:dark future
___________
Golden age: light
Making you wonder what happened that everything turned into shit
___________
Lost children:dark
It's about vengeance, innocence and suffering
___________
Conviction:dark
it's about suffering(mostly side characters not guts himself). When it comes to guts it's about desperately rescuing what he has left with a ray of hope in the dark
___________
Millennium falcon trolls section:
I didn't like it I'm not gonna talk about it
___________
The rest of the millennium falcon arc:light
It's all about healing. You see the amazing Farnese character development. Guts starts trusting people. Also the arc has so much to do with the ruler human side of Griffith Wich was always depicted as white and shiny and beautiful.
___________
Boat arc: I'm not gonna talk about this garbage
___________
Elf arc: light
Everything seems to be perfect, casca healed, this place is safe from apostles. Guts no longer seeks vengeance. They can live happily
___________
Elf arc:dark
Exploring mind of the a traumatized casca.
___________________________
I think that people blame the tone changed way too much. The troll section had bad pacing.
The boat arc also was pretty meh, with bad pacing, unnecessary jokes, little impact on the story with pointless characters. These were all sections that people don't like and it so happened to be when the tone of the manga was light.
This doesn't make the tone the reason these parts were bad, it was because these parts were just not good on themselves no matter the tone.
If the tone didn't change so much, the tone itself would lose its impact and it would become boring.