Fix an Issue where you can't set the DatePicker to custom background (#2518)

Author: ahmed-abdelrazek

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.DatePicker.xaml

@@ -459,6 +459,7 @@
                             x:Name="PART_TextBox"
                             Grid.Column="0"
                             Grid.Row="0"
+                            Background="{TemplateBinding Background}"
                             Focusable="{TemplateBinding Focusable}"
                             Style="{StaticResource MaterialDesignDatePickerTextBox}"
                             VerticalContentAlignment="Center"