Set background to constant color

Might make more sense to not use GridBackground (though it’s a great
example of custom style properties!)

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Styles/MaterialGraph.uss

 	width: 200;
 }
 
-MaterialGraphView {
+GridBackground {
 	grid-background-color:rgb(20, 21, 21);
 	line-color: rgb(20, 21, 21);
 	thick-line-color: rgb(20, 21, 21);