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Dark Mode is available with a subscription, although I'm not sure which level it is activated, but I'd guess the first level since it's a basic feature.
Dark mode is included for everyone. It is all the way at the bottom, and probably says "TMC 2016", if you click on that you get to select from the 4 styles.
If you scroll all the way down to the bottom you will see the option you currently have selected, and you can click on that to change between normal and dark mode.