If it's just dark I don't think that would necessarily not let it get featured. Believe it not pantysmile's memes didn't violate the Terms of Service, they just needed to be flagged NSFW.
I've also figured out another thing that isn't mentioned in the Posting Guidelines that causes memes to not get featured. The best example for this is a comic strip. The entire joke is already on the picture that gets uploaded. You don't add any, or minimal, text to it. Other users can't reuse that template because the whole joke is already there. Those don't get featured as memes. If you do them as Demotivationals and figured a caption for it, that's one way to get around that.
If you ever have one that doesn't feature and you can't figure out why, feel free to send me a link to it and I can give my best interpretation and at least lend an upvote to see if that helps.