A new type of Pokémon introduced in Luna, Terra, and Sol Versions is the Light-Type. It could be considered the counterpart to Dark-Types. Light-Type Pokémon are generally brightly colored and have some source of luminescence on their body. Their attacks normally involve the manipulation of light. Light-Type Pokémon are powered up when in the Harsh Sunlight weather condition, but weakened under the Full Moon.
Light is super effective against Dark (Lights up the dark), Ghost (ghosts don’t like light), and Psychic (light disrupts focus).
Types that are super effective against Light: Dark (darkness shrouds light), Grass (plants use light energy to make food), and Steel (polished steel reflects light).
Light is not very effective against Light (produces even more light), Grass, and Steel
Types that are not very effective against Light: Electric (produces light) and Fire (produces light), and Light.
Type that have no effect against Light: Fairy (Fairies attack with light energy)
So yes, Light and Dark-Types are super effective against each other. The closet comparison that exists to this already is Dragon being super effective against itself.
Old Pokémon That become Light-Types:
Volbeat and Illumise become Bug/Light-Types.
Mareep, Flaaffy, Ampharos, Luxio, Luxray, Helioptile, Heliolisk, and Xurkitree become Electric/Light-Types.
Sunflora, Morelull, and Shiinotic become Grass/Light-Types.
Watchog becomes a Normal/Light-Type.
Porygon, Porygon2, Porygon-Z become pure Light-Types.
Staryu, Starmie, and Lumineon become Water/Light-Types.
Necrozma (Normal Form) becomes Light/Psychic-Type
Ultra Necrozma becomes Light/Dragon-Type.
Arceus gains a Light-Type form using the Sun Plate.
Rotom gains a Lamp Form that is an Electric/Light-Type.
Castform’s Sunny Form is now a Light-Type, and Castform learns the moves Strobe and Solar Beam instead of Ember and Fire Blast respectively.
Chinchou and Lanturn do not become Light-Types because to remove either the Water or Electric-Type from them would change them too much, instead they are given the new ability Luminate, which turns their Normal-Type moves to Light-Type moves, as well as giving a 50% boost to all Light-Type moves used, so they might as well be Light-Types.
Old Moves that become Light-Type
Flash, Tail Glow, Sunny Day, Morning Sun, Swift, Light Screen, Mirror Coat, Spotlight, and Prismatic Laser, Photon Geyser, and Light the Burns the Sky.
Weather Ball now becomes a Light-Type move when used during Harsh Sunlight.
Moves that are now a Light Combo Type
(Moves can now be two types at the same time, think Flying Press)
Signal Beam (Bug/Light), Dazzling Gleam (Fairy/Light), Aurora Beam (Ice/Light), Power Gem (Rock/Light), Flash Cannon (Steel/Light), and Aurora Veil (Ice/Light).
In addition, Solar Beam, when used by a Light-Type Pokémon, becomes a Light-Type move, otherwise it’s still Grass-Type.