(Before you ask, this is meant as a NON-SPOILER post for those who are not sure if they want to read or not & to see my opinion and recommendations)
It's a manga about kids, what can be dark about it? EVERYTHING OF COURSE.
So we're introduced to Cooro, an energetic +Anima who is looking to gather up other +Anima to form a gang. The difference from other gangs is that it's a group of friends, and nothing more. We meet people like Husky, a feminine looking guy who happens to be a sire-- I mean fish (Hey author, why in the world didn't you make him a girl?).. Anyways, we got the mysterious Senri who is a bear +Anima that rarely talks but cares for his friends, and Nana, a bat who is the only girl in the group (and you make Nana the girl of the story too author. Where you trying to break stereotypes here? owai--).
So you remember those dark themes? Oh, you've also seen the tags? Well yep, that's exactly the story. A fluffy light hearted story with themes of racism, hatred for different, using others for personal advantages (For those who read, I refer to Cooro's REAL purpose of the group). Still, it doesn't take away from the story. Everyone's purpose is pure, and through each individual character arc we further understand why characters act like they do.
Enough talking, here's a summary (+ = positive, - = negative):
+Good story with great character development
+Dark undertones & themes which viewers who are into plot can easily identify
+Group of friends doing stuff friends do, no lovey-dovey stuff here
-The ending. They should have expanded more on what the hell was going on on the last few chapters.
For recommendations, I'll have to go to something a bit far-fetched (I've only started recording the mangas I've read recently) Defense Devil. Similar themes of people who are looked as evil and bad being the complete opposite.