A splendidly-named Civilization modder, GrrrArrrg, has released an environment reskin for Civilization VI. It tunes down the colour saturation and modifies many terrain textures to recreate the look of Civilization V.
When it was first revealed, Civilization VI’s art style was pretty controversial. Featuring warm colours and cartoonishly-proportioned character models, its look seemed inspired by the Civilization Revolution mobile spin-offs, and alienated Civ’s more traditional fan-base.
If that’s you, then you’ll want to check out Environment Skin: Sid Meier’s Civilization V on the Steam Workshop. It’s compatible with every expansion, up to and including Gathering Storm. GrrrArrrg has done a great job recreating the look of Civ V, through a more muted colour key and modifications to all terrain textures. They’ve even adjusted the density of water (modders have some godlike powers these days) and the base colour of the ocean floor “to match [the] lighter sand of Civ V.”