Fix: make HistogramLUTWidget obey default background color

pyqtgraph/widgets/HistogramLUTWidget.py

@@ -13,7 +13,7 @@ __all__ = ['HistogramLUTWidget']
 class HistogramLUTWidget(GraphicsView):
     
     def __init__(self, parent=None,  *args, **kargs):
-        background = kargs.get('background', 'k')
+        background = kargs.get('background', 'default')
         GraphicsView.__init__(self, parent, useOpenGL=False, background=background)
         self.item = HistogramLUTItem(*args, **kargs)
         self.setCentralItem(self.item)