Pic related is how RGUI used to look like at its inception. You can still use it today, but it's no longer the default, and if you do try it, it has at least been made to look less terrible from the get-go, and you can customize it any number of ways.

For an extra fun insight into what went on in the minds of the RetroArch team back then in terms of UI development, here's a nice little thread where, as usually tended to happen, got rather hostile and then locked once RetroArch's current lead dev got involved:
https://forums.libretro.com/t/phoenix-gui/368/19