Dark Mode FAQ
Where are the settings?
In the Options menu: the Dark Mode button in the top left.
The first loading window was still red.
On Forge and NeoForge, the first window opens before any mod runs, so it's red the very first time you start with Dark Mode. It's black from the next start on. That window only knows black or red, so other loading colors begin once the game's own loading screen appears.
A screen from another mod stayed light or looks wrong.
Screens are recognised by their texture names, so one with an unusual name can stay light, and a screen drawn in an unusual way can look off. If it looks wrong, add the mod's id under Mods to leave alone to give it its normal colors back.
Does the server need it?
No, it's client-side. It also does no harm on a server, so a shared modpack is fine.
Does it work with resource packs?
A pack that retextures screens gets darkened on top of its own colors. If you already use a dark GUI pack, the result can be too dark: try a higher brightness, or remove the pack.
I removed the mod and the loading screen is still black.
Dark Mode turns on vanilla's Monochrome Logo option so the first loading window goes dark. That option stays on after you remove the mod. Turn it off in Accessibility Settings. Inside the mod, picking Red or turning Dark Mode off switches it back for you.