You can absolutely write specifications for art. And I would consider a pretty technical checklist for quality control that is agreed on in advance (lighting fr

You can absolutely write specifications for art. And I would consider a pretty technical checklist for quality control that is agreed on in advance (lighting from predetermined direction, adjerence to color palette for corresponding scenery, shadows consistent, required silhouettes recognizable etc.). I remember an old style guide written by MS for Windows XP that outlined in great detail how icons in the default theme style meed achieve their look. It gave technical details like the exact location of the vanishing point for perspectively drawn icons. So coming up with an art style specification should be doable once the style has been developed.