Comment by Tmpod
Well, I can't check if it is still a thing today, but Element's managed servers used to allow you to do that, at least for the web app (though I had the idea they also did that for the old mobile apps?)
Here's a couple of links:
https://element.io/blog/custom-branding/
https://element.io/blog/a-white-label-messaging-app-to-creat...
They've been on their game with this product, which hadn't been on my radar until Aaron's response above.