Im planning on having a darker theme/story or both. Does the 18+ restriction apply to this or is it just NSFW, if it does can I get some examples or some more details?

The prohibition of 18+ topics, in my understanding, means that in your product, in addition to what you have already written, there should be no propaganda of narcotic drugs, suicides and other darkies (that is, a ban on antisocial behavior). I think it is better to avoid topics that can cause psychological harm to the child. As for your question, we need more details about how dark the game will be, I think that if we do not touch on the previously mentioned topics, then everything should be fine.