A saturation slider, or something,[...]
You can't change art-style and/or design with a slider. For example, WoW style is quite distinguishable, and sometimes this unique aspect of it being used for comparison, to describe other media: recently dead Copernicus has quite similar "style" to WoW, so is latest trailer for the "Guardian of Middle Earth".
As for simple saturation changes- you can use PowerStrip's personal profile for your DivinityOriginalSin.exe.
