Use vivid and descriptive language to bring out the darkness. Instead of just saying it's dark, describe the shadows as if they are alive, the coldness that seeps into the bones of the characters. Build up tension gradually. Maybe start with a small, strange event in 2017 and then slowly escalate it until the characters are fully immersed in a nightmarish situation.
To write a good 'dark 2017 fanfiction', first, create a really moody and atmospheric setting. Since it's related to 2017, you could incorporate real - world events or trends from that year in a dark and twisted way. For example, if there was a big technological trend in 2017, make it into something that has a negative impact on the characters.
Develop complex and flawed characters. In a 'dark 2017 fanfiction', the characters should have their own demons. They could be dealing with past traumas or current situations that are pushing them towards the dark side. Show their internal struggles and how they change over time in this dark setting. Also, don't be afraid to introduce moral ambiguity. The line between right and wrong should be blurred in a good dark story.