Nah... It was $10 all along, then after the Dark theme got out of Beta it was available for everyone, then I changed my Patreon down to $5 (because of... stinginess?). Well, too bad. No black mode for me for now.
I think all backers had access to the dark theme, even $2 backers, which was probably a mistake (no separation between the different tiers in the forum software I guess?), I'm very grateful for that though since I'm a $5 backer.
There must be some separation of the tiers since I'm a $2 backer and get ads on the site like I'm supposed to, but I don't know how it works for the themes