WhiteErase/WriteErase/ClassStyle/StyleComboBox.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <ControlTemplate x:Key="ComboBoxToggleButton" TargetType="{x:Type ToggleButton}">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
                <ColumnDefinition Width="30"/>
            </Grid.ColumnDefinitions>
            <Border x:Name="Border" 
                    Grid.ColumnSpan="2"
                    CornerRadius="5"
                    BorderBrush="#59A862" 
                    BorderThickness="1"
                    SnapsToDevicePixels="true">
            </Border>
            <Border Grid.Column="1" 
                    BorderBrush="#444" Name="ButtonBorder"
                    CornerRadius="0" BorderThickness="0" 
                    Background="Transparent" />
            <Border Grid.Column="1">
                <Path x:Name="Arrow" 
                      Data="F1 M 0,0 L 2.667,2.66665 L 5.3334,0 L 5.3334,-1.78168 L 2.6667,0.88501 L0,-1.78168 L0,0 Z" 
                      Fill="#59A862" 
                      Stretch="Fill"
                      Height="7"
                      Width="10"
                      HorizontalAlignment="Center"
                      VerticalAlignment="Center"/>
            </Border>
        </Grid>
        <ControlTemplate.Triggers>
            <Trigger Property="UIElement.IsMouseOver" Value="True">
                <Setter Property="Panel.Background" TargetName="ButtonBorder" Value="Transparent"/>
            </Trigger>
            <Trigger Property="ToggleButton.IsChecked" Value="True">
                <Setter Property="Panel.Background" TargetName="ButtonBorder" Value="Transparent"/>
                <Setter Property="Shape.Fill" TargetName="Arrow" Value="#498C51"/>
            </Trigger>
            <Trigger Property="UIElement.IsEnabled" Value="False">
                <Setter Property="Panel.Background" TargetName="Border" Value="#498C51"/>
                <Setter Property="Panel.Background" TargetName="ButtonBorder" Value="#498C51"/>
                <Setter Property="Border.BorderBrush" TargetName="ButtonBorder" Value="#498C51"/>
                <Setter Property="TextElement.Foreground" Value="#498C51"/>
                <Setter Property="Shape.Fill" TargetName="Arrow" Value="#498C51"/>
            </Trigger>
        </ControlTemplate.Triggers>
    </ControlTemplate>

    <ControlTemplate x:Key="ComboBoxTextBox" TargetType="{x:Type TextBox}">
        <Border x:Name="PART_ContentHost"
                Focusable="True"
                Background="{TemplateBinding Background}"/>
    </ControlTemplate>

    <Style TargetType="ComboBox">
        <Setter Property="UIElement.SnapsToDevicePixels" Value="True"/>
        <Setter Property="FrameworkElement.OverridesDefaultStyle" Value="True"/>
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
        <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
        <Setter Property="ScrollViewer.CanContentScroll" Value="True"/>
        <Setter Property="FrameworkElement.FocusVisualStyle" Value="{x:Null}"/>
        <Setter Property="Height" Value="40"/>
        <Setter Property="Width" Value="250"/>
        <Setter Property="FontSize" Value="14"/>
        <Setter Property="Foreground" Value="#59A862"/>
        <Setter Property="HorizontalAlignment" Value="Center"/>
        <Setter Property="Margin" Value="0 10 0 0"/>
        <Setter Property="Cursor" Value="Hand"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="ComboBox">
                    <Grid>
                        <ToggleButton x:Name="ToggleButton"
                                      Grid.Column="2"
                                    ClickMode="Press" Focusable="False"
                                    IsChecked="{Binding Path=IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}"
                                    Template="{StaticResource ComboBoxToggleButton}"/>

                        <ContentPresenter Name="ContentSite" 
                                          Margin="5 0"
                                          IsHitTestVisible="False"
                                          HorizontalAlignment="Left" VerticalAlignment="Center"                              
                                          Content="{TemplateBinding ComboBox.SelectionBoxItem}" 
                                          ContentTemplate="{TemplateBinding ComboBox.SelectionBoxItemTemplate}"
                                          ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"/>
                        <TextBox Name="PART_EditableTextBox" Margin="3 10"                     
                                          IsReadOnly="{TemplateBinding IsReadOnly}"
                                          Visibility="Hidden" Background="Transparent"
                                          HorizontalAlignment="Left" VerticalAlignment="Center"
                                          Focusable="True">
                            <TextBox.Template>
                                <ControlTemplate TargetType="TextBox" >
                                    <Border Name="PART_ContentHost" Focusable="False" VerticalAlignment="Center"/>
                                </ControlTemplate>
                            </TextBox.Template>
                        </TextBox>
                        <Popup Name="Popup" Placement="Bottom"
                              Focusable="False" AllowsTransparency="True"
                              IsOpen="{TemplateBinding ComboBox.IsDropDownOpen}"
                              PopupAnimation="Slide">
                            <Grid Name="DropDown" SnapsToDevicePixels="True"
                                  MinWidth="{TemplateBinding FrameworkElement.ActualWidth}"
                                  MaxHeight="{TemplateBinding ComboBox.MaxDropDownHeight}">
                                <Border Name="DropDownBorder" 
                                        Background="White" Margin="1"
                                        CornerRadius="0" BorderThickness="0" 
                                        BorderBrush="#498C51"/>
                                <ScrollViewer Margin="4" SnapsToDevicePixels="True">
                                    <ItemsPresenter KeyboardNavigation.DirectionalNavigation="Contained" />
                                </ScrollViewer>
                            </Grid>
                        </Popup>
                        <Label x:Name="Watermark"
                               VerticalAlignment="Center"
                               HorizontalAlignment="Left"
                               Foreground="#59A862"
                               Visibility="Collapsed"
                               Padding="10 0 0 0"
                               Content="{TemplateBinding Tag}"
                               FontSize="15"/>                        
                    </Grid>
                    <ControlTemplate.Triggers>
                        <MultiTrigger>
                            <MultiTrigger.Conditions>
                                <Condition Property="Text" Value=""/>
                            </MultiTrigger.Conditions>
                            <Setter Property="Visibility" Value="Visible" TargetName="Watermark"/>
                        </MultiTrigger>
                        <Trigger Property="ItemsControl.HasItems" Value="False">
                            <Setter Property="FrameworkElement.MinHeight" TargetName="DropDownBorder" Value="95"/>
                        </Trigger>
                        <Trigger Property="UIElement.IsEnabled" Value="False">
                            <Setter Property="TextElement.Foreground" Value="#498C51"/>
                        </Trigger>
                        <Trigger Property="ItemsControl.IsGrouping" Value="True">
                            <Setter Property="ScrollViewer.CanContentScroll" Value="False"/>
                        </Trigger>
                        <Trigger Property="ComboBox.IsEditable" Value="True">
                            <Setter Property="KeyboardNavigation.IsTabStop" Value="False"/>
                            <Setter Property="UIElement.Visibility" TargetName="PART_EditableTextBox" Value="Visible"/>
                            <Setter Property="UIElement.Visibility" TargetName="ContentSite" Value="Hidden"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>

    </Style>
</ResourceDictionary>