Dark and gothic and serious? You do realize this is the same gameworld where an avatar of pure evil got headbutted into unconsciousness by a giant orc, do you not?

LOL

Let's see. Extreme violence. Lots of explorations of madness. The RPG even warns the game is for mature audiences only in the front of the book. A literal demonic orgy at the beginning of another book. Explorations of the mind of a Chaos worshiper in another. Yet another orgy in another, but this one by elves. Exploration on themes of marriage. More explorations of death, friendship, sadism, gore, evil, incest, rape, cannabilism. One book is basically even Silence of the Lambs in the Warhammer setting.

So yes, dark, Goth, serious. Perhaps you're not familiar enough with the setting to realize this.

You are right the lore and the pictures are usually very gothic. On the other hand the miniatures themselves, some novels, some specialist games etc. are very comic.

So even Games Workshop tries to appeal a wider audience with different aspects. The models of the game are pretty much taken straight from the TT.