MAINT: fix background color of parameter boxes

In py3 wxpython, background color is not white even if no constrains applied.

Author: dragonyanglong

src/diffpy/pdfgui/gui/phaseconfigurepanel.py

@@ -266,7 +266,8 @@ def restrictConstrainedParameters(self):
                 tt.SetTip(self.constraints[var].formula)
             else:
                 textCtrl.SetEditable(True)
-                textCtrl.SetBackgroundColour(txtbg)
+                #textCtrl.SetBackgroundColour(txtbg)
+                textCtrl.SetBackgroundColour(wx.WHITE)
 
         # Now the grid
         rows = self.gridAtoms.GetNumberRows()