Hi,
here my little demo. It works without problems.
<Application x:Class="Application"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:PIM"
xmlns:xctk="clr-namespace:WpfControlLibrary1;assembly=WpfControlLibrary1"
StartupUri="MainWindow.xaml">
<Application.Resources>
<Style
x:Key="DateTimePickerBase"
TargetType="{x:Type xctk:DateTimePicker}">
</Style>
<Style
x:Key="LabelBase"
TargetType="Label">
</Style>
</Application.Resources>
</Application>
<Window x:Class="MainWindow"
xmlns:self="clr-namespace:PIM"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:PIM"
xmlns:xctk="clr-namespace:WpfControlLibrary1;assembly=WpfControlLibrary1"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<Window.DataContext>
<self:MainWindowVM />
</Window.DataContext>
<Window.Resources>
<Style
BasedOn="{StaticResource DateTimePickerBase}"
TargetType="{x:Type xctk:DateTimePicker}">
</Style>
<Style
BasedOn="{StaticResource LabelBase}"
TargetType="Label">
</Style>
</Window.Resources>
<Grid>
</Grid>
</Window>