HUDReplacer
HUDReplacer is a framework that allows you to replace the HUD/UI textures at runtime, without the hassle of modifying any stock files or assets.
The mod is currently a Work in Progress, so bugs may occur. It does not come with any custom textures out of the box, though an example config can be found on the github page for those who want to create their own.
This is intended as a dependency and a tool for modders to more easily create and share texture packs.