There were many artists who worked on concepts and actual designs in Project Spark from the early drafts to the finished article. Here are some of the highly talented people who created the much maligned, but very distinctive look of Project Spark (or in some cases the rejected looks).
Lead Concept Designer at Microsoft Studios. Responsible for the box art and the main menu design.
Senior Character Artist. Created some of the the 3D Models for character design in Project Spark
Ryan Scott Lee
UI Artist. Worked with Team Dakota in 2013 to design the brain tiles in Project Spark and the early menu designs that were used during Alpha and early Beta.
Conceptual Design Artist. Worked on the early concepts for Project Spark.
3D Character Artist. Created some of the character models for Project Spark.
AtomHawk Design Team
Concept art for Project Spark
Made the second E3 launch trailer for Project Spark. It is featured in their showreel.