In Bergen Town, the bergens are singing "Clint Eastwood" by Gorillaz. Again, people complain the song is ruined because it's not necessarily a song about being depressed or miserable. But in the same vein, that maybe the point: the bergens have such bad attitudes that it downs even the best songs.
Plus, there's the double meaning that they really are "feeling glad", because they know they'll get to eat trolls soon. "The future is coming on" indicates they're anticipating the moment they get to eat the trolls and (if temporarily) experience happiness.
There's also one line in the song that foreshadows that the Bergens are capable of being happy without eating the trolls: "And remember that it's all in your head..."
Later, after Branch's backstory about how his grandmother was stolen by a Bergen, all of the other trolls (and Brigette) sing the lyrics to "Total Eclipse of the Heart". One would wonder how happy trolls would know such a sad song. But Poppy is the only one who doesn't sing the song, perhaps unfamiliar with it. This hints that, despite being the future leader of the trolls, Poppy is the only troll who truly doesn't know what it feels to be unhappy.
Perhaps Branch knew that song as a kid because his Grandmother taught it to him. And she knows this sad song because it's how she felt when she lost her child (one of Branch's parents) to the bergens.
Finally when Branch sings "True Colors" to restore Poppy's color, one can say this is finally a fitting song on so many levels. Not just because it's about Branch seeing Poppy's true colors. It's also both a happy and poignant song about accepting each other for who they are. Branch has accepted Poppy for being up-beat, and she accepts that he likes sad gentle moments.
When we’re first introduced to King Gristle Jr., “In the Hall of the Mountain King” is playing. Bergen is German for “mountains”, so they are literally in the Hall of the Mountain King. Funnily enough, the Mountain King the original song refers to is a Troll.
What happened to Poppy's mom? If it's only fitting for the Bergen prince to have a troll princess, then for the Bergen king... oh dear.
Applies to Branch’s parents as well, along with most of the other troll families, if every Bergen ate a troll at every Trollstice, who knows what the casualty numbers were over the years.
There are several mentions of Poppy seeing the world as "cupcakes and rainbows", but earlier we see that trolls poop cupcakes and bleed rainbows (the latter perhaps figuratively). So either her worldview is so unfailingly optimistic that she literally can't see unpleasant things, or it's a bit darker than she lets on.
One for the Bergens: when Trollstice is reinstated, Chef plans to serve every troll at the dinner. Given that her political ambitions hinge completely on supplying the Bergens with trolls, and the Bergens believe that they can only attain happiness by eating the trolls, perhaps wiping out the entire troll population in one night isn't a very bright idea.
Then again, doesn't it reflect human folly how we hunt and eat certain animals to the point of endangerment?
They might possibly create a system where they have troll farms to so they can harvest them. Not that that's any better.
Or re-create. Who said the place the trolls escaped from wasn't seen by the bergens as a troll farm?