This is a very good question
In the cats and dogs stream, it is relatively straight forward to decide whether to feature a meme. They are cats or they're dogs
Dark humour however, is far more subjective, by which I mean it is down to the moderator who sees it. What I find dark does not necessarily mean some other mod will feel the same
imgflip.com/i/9wjy4p
I wrote this some time ago to give examples of what I find dark - I hope this helps