Hi,
What I can tell is that v2.5 of the toolkit has really changed the TimePicker (providing new base class and new properties). Since you are using Windows7, the "Generic.xaml" files will be used as default template for toolkit's controls.
When lookking at the template "WatermarkTextBox1" that you have, the Border (x:Name="Border") should have a Background of "TemplateBinding Background". But in the ControlTemplate.Triggers, if IsEnabled = False, the Border's Background is set to #FFF4F4F4 ; which correspond to what you have in WaterMarkTextBox XAML from XAML Spy.
Maybe just removing this setter, from triggers, would resolve the issue, without setting Opacty of Border to 0.
What I can tell is that v2.5 of the toolkit has really changed the TimePicker (providing new base class and new properties). Since you are using Windows7, the "Generic.xaml" files will be used as default template for toolkit's controls.
When lookking at the template "WatermarkTextBox1" that you have, the Border (x:Name="Border") should have a Background of "TemplateBinding Background". But in the ControlTemplate.Triggers, if IsEnabled = False, the Border's Background is set to #FFF4F4F4 ; which correspond to what you have in WaterMarkTextBox XAML from XAML Spy.
Maybe just removing this setter, from triggers, would resolve the issue, without setting Opacty of Border to 0.