#1140 Background paint color changes automatically

[Srijan Kumar]

When the default background paint (under right-click - properties - other) color is black when theme JFree is selected, and then the theme is switched to JFree/Shadow and back to JFree, the background paint color changes to white. This happens in, at least, PieChartDemo1 and RingChartDemo2, in JFreeChart 1.0.19 Demo file.