Many messaging apps like Signal and Telegram have slight shading and outlining across their UI and messages. These little details make things look much cleaner and more visually appealing, as well as making it seem less cheap and simple.

Even as an option in the settings, subtle visual improvements like these improve the experience pretty greatly. Looking professional and clean goes a long way.

And, to take it a step further, what if Textra adopted an option for gradient coloring, like Signal? On Signal, when you're customizing a conversation you can simply choose a color or you can choose two colors to create a little gradient.
The difference between Textra and Signal is that you can only choose one color/gradient for a conversation, and the user's messages stay gray regardless (not to mention that different people can't have unique colors in group chats on Signal). But on Textra the conversation is much more customizable, allowing for contact specific coloring. However, Textra has a disadvantage in that it looks quite flat and doesn't have gradients (I won't get into the fact that we can only choose from a set list of colors, rather than a hex code or color wheel).