Does anyone know how to collaborate with devRant? I mean some improvements in the UI for instance. Is there a repo?

The age old question.

There is a repo on GitHub, but there is no collaboration or open source code.

The official clients and the website development has been abandoned a looong time ago. The owners of devrant only keep the servers alive and that‘s it. We are not allowed to make our own apps and publish them in the official app stores.
There are many inofficial devrant clients out there. For instance, I have one for iOS called JoyRant.
There is a public REST api for devRant that we can use but it‘s undocumented and incomplete.