Why is it an issue though?
In the other thread you made, everyone is completely stumped on trying to even think of a story or show that doesn't deal with something heavy, like death, emotional pain, grief, abuse, fear... The most people have managed are very light "slice of life" series about people in inconsequential situations, like Azumanga Daioh, which is one of my favourite comics ever... but it barely has a story; it's little snapshots of funny things that happen to a bunch of girls at school.
Stories that really have meaning tend to be about overcoming a problem, and for a problem to feel big enough that the tale of how that person dealt with (or sometimes, failed to deal with) it merits telling a story about it and putting that story in print and sharing it wih the world... the problem generally needs to be something big. It needs to be about dealing with heartbreak, despair, the fear of death, the pain of loss.
A story can still be fun, happy and enjoyable for even small children while touching on these things. Just look at Finding Nemo; a movie about a widower whose whole family were killed and now his disabled son is lost somewhere and he must overcome deadly peril to find him. It's a fun movie! For kids! Or My Neighbour Totoro, where two girls move to the countryside because their mother is dealing with a bad illness, and at one point everyone in the neighbourhood thinks one of them died and they go out looking, terrified they might find a body... it's a whimsical, magical children's movie!
The problem is never having dark themes, it's always exploring them in a way appropriate for the intended tone and audience.