Add custom background color to 02 tutorial config

apps/tutorials/pe/02_ConfinedGasExtended.cfg

@@ -21,6 +21,7 @@ Raytracing
    upVector < 0, 0, 1 >;
    antiAliasFactor 2;
    reductionMethod MPI_REDUCE;
+   backgroundColor < 0.18, 0.18, 0.18 >;
    Lighting {
       pointLightOrigin < -10, 10, 15 >;
       ambientColor < 0.3, 0.3, 0.3 >;