Dark Mode Everywhere uses the built in 1.18 Minecraft shaders to transform any GUI texture into a Dark Mode. Thanks to the shaders it will work in any GUI in the game, it will also try to modify the text color is its not hard to read in dark mode. Please report any visual issues so I can fix it.
The mod currently provides with 3 dark modes to suit your needs that can be easily changed by clicking on the button on the bottom left:
You can have the possibilty if adding new GUI shaders by modifying the "darkmodeeverywhereshaders.json" and using a Resource Loader mod or a Resource Pack.