WPF

<MenuItem
    Command="{Binding Path=DataContext.MenuItemSelectedCommand, ElementName=layoutRoot}"
    CommandParameter="{Binding}"
/>

1. In project #2, add a reference to project #3;
2. Define styles in project #2 for the controls in project #3, in exactly the same way you define styles for the built-in controls;

1. So how does the app tell the control what style to use? The Telerik assembly doesn't know about your app.
2. How do you tell specific controls in that library what to look like. It's not enough to set the TargetType because that will set ALL controls of that type. You would need a way to say "For this specific combobox, make it look like this".

Kevin Marois wrote:

If in the User Control library I created a reference to the app, that makes that User Control library bound forever to that one app.

Kevin Marois wrote:

There has to be a way in the form it's on to say "Set the combo boxes to use TimerPickerComboStyle" ...

public static readonly DependencyProperty ComboBoxStyleProperty = DependencyProperty.Register(
    "ComboBoxStyle", typeof(Style), typeof(TimePicker));

public Style ComboBoxStyle
{
    get { return (Style)GetValue(ComboBoxStyleProperty); }
    set { SetValue(ComboBoxStyleProperty, value); }
}

private BindingBase GetTimePickerBinding(DependencyProperty property)
{
    return new Binding(property.Name)
    {
        Source = this
    };
}
 
public override void OnApplyTemplate()
{
    base.OnApplyTemplate();
    
    var comboBox = GetTemplateChild("PART_FirstCombo") as ComboBox;
    if (comboBox != null)
    {
        comboBox.SetBinding(FrameworkElement.StyleProperty, 
            GetTimePickerBinding(ComboBoxStyleProperty));
    }
}

<uc1:TimePicker
    ComboBoxStyle="{StaticResource TimePickerComboStyle}"
/>

<Style TargetType="{x:Type uc1:TimePicker}">
    <Setter Property="ComboBoxStyle" Value="{StaticResource TimePickerComboStyle}" />
</Style>

Richard Deeming wrote:

Or in a global style within the application:

XAML

<Style TargetType="{x:Type uc1:TimePicker}">
    <Setter Property="ComboBoxStyle" Value="{StaticResource TimePickerComboStyle}" />
</Style>

public partial class UserControl1 : UserControl
{
    public static readonly DependencyProperty IsShowHideVisibleProperty = DependencyProperty.Register(
        "IsShowHideVisible", typeof(bool), typeof(UserControl1),
        new FrameworkPropertyMetaData(false));

    public bool IsShowHideVisible
    {
        get { return (bool)GetValue(IsShowHideVisibleProperty); }
        set { SetValue(IsShowHideVisibleProperty, value); }
    }

    public UserControl1()
    {
        InitializeComponent();
        DataContext = this;
    }
}

<Expander Grid.Row="1"
            Grid.Column="2"
            x:Name="expander"
            BorderThickness="1" 
            BorderBrush="SteelBlue" 
            Background="White"
            ExpandDirection="Down"
            IsExpanded="False"
            HorizontalAlignment="Right"
            Margin="0,0,5,7">

<pre>
<Expander.Effect>
    <DropShadowEffect/>
</Expander.Effect>

<Expander.Style>
    <Style  TargetType="Expander">
        <Setter Property="Header" Value="My Stuff"/>
        <Style.Triggers>
            <Trigger Property="IsExpanded" Value="False">
                <Setter Property="Header" Value=""/>
            </Trigger>
        </Style.Triggers>
    </Style>
</Expander.Style>

<Expander.Header>
    <TextBlock Text="Sensors">

        <TextBlock.Style>
            <Style TargetType="TextBlock">
                <Style.Triggers>
                    <DataTrigger Binding="{Binding Path=IsExpanded, RelativeSource={RelativeSource TemplatedParent}}"
                                    Value="False">
                        <Setter Property="LayoutTransform">
                            <Setter.Value>
                                <RotateTransform Angle="90"/>
                            </Setter.Value>
                        </Setter>
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </TextBlock.Style>

    </TextBlock>
</Expander.Header>

<f6cc:FalconSensorList Width="150"/>

<Expander x:Name="MyExpander" BorderThickness="1"  BorderBrush="SteelBlue"
        Background="White" IsExpanded="False" HorizontalAlignment="Right" MaxWidth="150" Margin="0,0,5,5">
    <Expander.Style>
        <Style>
            <Style.Triggers>
                <DataTrigger Binding="{Binding ElementName=MyExpander, Path=IsExpanded}" Value="False">
                    <Setter Property="Expander.ExpandDirection">
                        <Setter.Value>
                            Left
                        </Setter.Value>
                    </Setter>
                </DataTrigger>
                <DataTrigger Binding="{Binding ElementName=MyExpander, Path=IsExpanded}" Value="True">
                    <Setter Property="Expander.ExpandDirection">
                        <Setter.Value>
                            Down
                        </Setter.Value>
                    </Setter>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Expander.Style>
    <Expander.Header>
        <TextBlock Text="My Stuff">
            <TextBlock.Style>
                <Style>
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding ElementName=MyExpander, Path=IsExpanded}" Value="True">
                            <Setter Property="TextBlock.LayoutTransform">
                                <Setter.Value>
                                    <RotateTransform Angle="0"></RotateTransform>
                                </Setter.Value>
                            </Setter>
                        </DataTrigger>
                        <DataTrigger Binding="{Binding ElementName=MyExpander, Path=IsExpanded}" Value="False">
                            <Setter Property="TextBlock.LayoutTransform">
                                <Setter.Value>
                                    <RotateTransform Angle="90"></RotateTransform>
                                </Setter.Value>
                            </Setter>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </TextBlock.Style>
        </TextBlock>
    </Expander.Header>
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="*"/>
            </Grid.RowDefinitions>
            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="*"/>
                    <ColumnDefinition Width="Auto"/>
                </Grid.ColumnDefinitions>
                <ComboBox Grid.Column="0"
                    Margin="2"/>
            </Grid>
            <ListBox Grid.Row="1"
                MinWidth="250"
                ItemsSource="{Binding Sensors, Mode=TwoWay}"
                ScrollViewer.VerticalScrollBarVisibility="Auto">
            </ListBox>
        </Grid>
    </Expander>