<Style x:Key="MyCustomStyle" TargetType="ComboBox">
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="ComboBox">
        <Grid>
          <VisualStateManager.VisualStateGroups>
            <VisualStateGroup x:Name="CommonStates">
              <VisualState x:Name="Disabled">
                <Storyboard>
                  <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Background">
                    <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ComboBoxDisabledBackgroundThemeBrush}"/>
                  </ObjectAnimationUsingKeyFrames>
                  <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Background">
                    <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ComboBoxDisabledBorderThemeBrush}"/>
                  </ObjectAnimationUsingKeyFrames>
                  <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
                    <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ComboBoxDisabledForegroundThemeBrush}"/>
                  </ObjectAnimationUsingKeyFrames>
                  <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="DropDownGlyph">
                    <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ComboBoxArrowDisabledForegroundThemeBrush}"/>
                  </ObjectAnimationUsingKeyFrames>
                </Storyboard>
              </VisualState>
            </VisualStateGroup>
          </VisualStateManager.VisualStateGroups>
          ...
          <ContentPresenter x:Name="ContentPresenter" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" Grid.Row="1" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
            <TextBlock x:Name="PlaceholderTextBlock" Foreground="#FFAAAAAA" FontWeight="Normal" Text="{TemplateBinding PlaceholderText}"/>
          </ContentPresenter>

.....
 <ContentPresenter x:Name="ContentPresenter"
               HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
               Margin="{TemplateBinding Padding}"
               Grid.Row="1"
               VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
    <TextBlock x:Name="PlaceholderTextBlock"
               Foreground="{ThemeResource ComboBoxPlaceholderTextForegroundThemeBrush}"
               FontWeight="{ThemeResource ComboBoxPlaceholderTextThemeFontWeight}"
               Text="{TemplateBinding PlaceholderText}" />
 </ContentPresenter>
 ....