To create ingame user interfaces, there are two options available:
ImGui is very easy to use. With just a few lines of code you can add a 2D user interface to your game. It is very limited in styling, though. It’s main purpose is either to mock up a UI quickly, or to expose developer tools.
RmlUi on the other hand is meant for building proper game UI, and therefore supports lots of styling options through CSS and and HTML like syntax for layouting. It is more work to set up, but gives you all the flexibilty you need.
It is recommended to use ImGui during early development and switch to RmlUi once your game reaches a more mature state. It is also possible to use both UI systems in parallel.