change to GradientTheme , added abstract GradientThemeBase to allow easier subclassing
change to GradientTheme , added abstract GradientThemeBase to allow easier subclassing

git-tfs-id: [https://tfs.codeplex.com/tfs/TFS01]$/SharpMap/Trunk;C60134

nhvu1988/SharpMap,SharpMap/SharpMap,nhvu1988/SharpMap,SharpMap/SharpMap