MainWindow.xaml

<Window x:Class="CheckListBox_Template.MainWindow"
        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:syncfusion="http://schemas.syncfusion.com/wpf"
        xmlns:local="clr-namespace:CheckListBox_Template"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        mc:Ignorable="d" WindowStartupLocation="CenterScreen"
        Title="Templates" Height="600" Width="500">
    <Grid>
        <syncfusion:CheckListBox  Grid.Row="1" 
            IsSelectAllEnabled="True" 
            Width="200" Height="370"
            HorizontalAlignment="Center" 
            VerticalAlignment="Center"
            x:Name="ListBox" ItemsSource="{Binding Vegetables}">
            <syncfusion:CheckListBox.DataContext>
                <local:ViewModel></local:ViewModel>
            </syncfusion:CheckListBox.DataContext>
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="Loaded">
                    <i:InvokeCommandAction Command="{Binding LoadedCommand}" />
                </i:EventTrigger>
            </i:Interaction.Triggers>
            
            <!--DataTemplate for the CheckListBox items-->
            <syncfusion:CheckListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock FontWeight="Bold" 
                                   Foreground="DeepSkyBlue" 
                                   Text="{Binding Name}" />
                    </StackPanel>
                </DataTemplate>
            </syncfusion:CheckListBox.ItemTemplate>
            
            <!--DataTemplate for the GroupHeader items-->
            <syncfusion:CheckListBox.GroupTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Name}"
                               Foreground="Green" 
                               FontWeight="Bold">
                    </TextBlock>
                </DataTemplate>
            </syncfusion:CheckListBox.GroupTemplate>

            <!--DataTemplate for the SelectAll items-->
            <syncfusion:CheckListBox.SelectAllTemplate>
                <DataTemplate>
                    <TextBlock FontWeight="Bold" 
                               Foreground="Red" 
                               Text="Select All" 
                               FontStyle="Italic"/>
                </DataTemplate>
            </syncfusion:CheckListBox.SelectAllTemplate>


        </syncfusion:CheckListBox>
    </Grid>
</Window>