Light mode during the day, dark mode at night
It’s Summit for Lemmy.
Play store link: https://play.google.com/store/apps/details?id=com.idunnololz.summit
There’s also m.lemmy.world running Voyager (formerly wefwef).
ActivityPub. It’s a protocol just like SMTP for email. It allows these different platforms you mentioned to interact with each other. They all together form the fediverse.
There’s gotta be something…