I'm sorry but as a person who's developed apps in Android in the past, I can tell you that making dark theme for apps should NOT be a paid feature whatsoever. This is purely and utterly ridiculous and clearly shows how corrupt Snapchat Inc. is. This is a basic feature that's been around in Android apps for a while now but they wanna act like it's the next big thing? Fuck off.

There's actually a way to install xpi files but it requires root unfortunately. There was a recent video posted on YouTube that shows modifications to the .json file in the data folder. If you're rooted definitely worth checking it out