It's a mod that lets you change the mouseover block highlight color, alpha, and line width on a global or per-block basis. Also lets you define an outline for materials the player cannot harvest (disabled by default).
That's it.
Thermonuclear_minecraft_engineer - Original idea and CurseForge icon
Q: How do I set it up?
A: Read the config.
Q: What if I can't read?
A: Then good luck reading these answers.
Q: Why do thick lines look funky near water?
A: Minecraft jank.
