I would like to propose to concentrate all development regarding the Look and Feel of Spark on JTattoo. Currently the code base includes Substance as second LaF provider. Substance is not maintened any more and causes functional bugs in Spark. This combination is IMHO reason enough to remove Substance from the code tree.
Together with the removal of Substance, the code should be reviewed for all hard coded images that break skining (lot's of light blue stuff around) and adapt the code for maximum efficency with respect to JTattoo.
One word about the selection process for JTattoo. This LaF was selected during the 2.6.0 development push after discovering the bugs caused by Substance. The old LaF Synthetica instisted on a commercial license and was subsequently removed from the code base. JTattoo provided several "good looking" skins that went well with the hard coded images and provided good support during the integration into 2.6.0.