The same way character customization in WoW are explained. They just exist. When Blizzard adds new ones they don't always create new lore to explain it. Tolkien never defined dwarven skin tones so why would it need an explanation? Arondir is a small issue though Tolkien did at one point describe an elf (Maegelin) as swarthy and dark-skinned but later wrote him as white. His early works had black=bad and white=good so depending on how he was writing characters it influenced how they looked. His later works generally avoided that association.
The world building for the adaptation is they just exist. No need to ask how or why. Tolkien's world would not change if Elves were blue, hobbits orange, and dwarves purple. Skin-tone is not a central part of the story.