Fancy Outlines

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.

Credits

Thermonuclear_minecraft_engineer - Original idea and CurseForge icon

FAQ

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.