I’m not a lawyer, but I’d say that’s a case for implied consent.
Typical example is when you’re shopping and you hand the cashier the money that they’re asking for, then that counts as an agreement to a contract. You don’t have to explicitly say that you’d like to buy the wares for that price.
With the dark mode button, I’d expect the same. You’re very likely cool with them storing your preference, specifically for providing you with dark mode (not for tracking et al). So, pressing the button would presumably suffice as consent for that.