Signal-You is a fork of a fork of the Android version of Signal that replaces the colors used in Signal with Material You ones (introduced in Android 12) that are based on your wallpaper. The original came from an issue on the upstream fork asking for a black theme for it, despite the maintainer saying that they weren't going to add it back in due to the code for the UI being changed too frequently.
What actually got me interested in trying it out was John Betaro pointing out that it's as simple as changing some XML values (at least I think it was him; the original user's account has been deleted since then...), which sounded much easier than I thought it could've been, so I gave it a shot and managed to get a version of the app with that black theme in there. However, that wasn't the end for this fork. I then started looking into how to incorporate Android's Material You colors into the app, and found out that I could implement them the same way, which led to this fork's first version! I also tried some other things such as adding in custom icons, but I didn't sick with those long, and eventually removed them since they weren't really adding much, and trying to figure out which files correlated to what icons was proving to be challenging.
As of today, the fork has gotten much more popular than I ever thought it would be. It's not perfect, though. For example, the settings menu has started using Signal's colors again due to them using Kotlin to make the page for that, and I haven't figured out how to get it to change from there, but for the most part, the colors still work! Maintaining this fork has given me some practice in using XML and Android Studio, and helped me learned some of the basics of Kotlin, which was a bit easier thanks to my college teaching Computer Science majors Java (even if you aren't going to be programming in the field you plan on pursuing).
Fun fact, actually: I used to build this app using GitHub runners due to the fact that the computer that I used to use for developing this fork wasn't up to the task of building it. Even though I considered it to be a really decent computer, especially given that the first number of times that I built the app locally were successful without any issues, I was proved very wrong whenever the entire computer would lock up trying to build the app, forcing me to forcefully shut it down, reboot it, and attempt to continue the build process. I've started doing that again, though, thanks to the new computer I got (thanks, Dad!), especially since it can finish the build much faster than the GitHub runners that I have access to will.