If your game isn't modded with BepinEx, DO THAT FIRST! Simply go to the latest BepinEx release and extract BepinEx_x64_VERSION.zip directly into your game's folder, then run the game once to install ...
This mod is a GUI library to ease the process of creating GUIs. Backported from 1.12.2 ModularUI. Requires GTNHMixins to work. This mod is under LGPL-3 license.