App.xaml.cs

<Application
    x:Class="App1.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:App1"
    RequestedTheme="Light">
    <Application.Resources>
        <!-- The custom color for the new placeholder -->
        <SolidColorBrush x:Key="MyCustomPlaceholderForegroundColor" Color="{StaticResource SystemBaseHighColor}"/>

        <!-- A copy of the ComboBox stlye with the custom placeholer color applied -->
        <Style x:Key="ComboBoxCustomPlaceholderStyle" TargetType="ComboBox">
            <Setter Property="Padding" Value="12,5,0,7"/>
            <Setter Property="MinWidth" Value="{ThemeResource ComboBoxThemeMinWidth}"/>
            <Setter Property="Foreground" Value="{ThemeResource SystemControlForegroundBaseHighBrush}"/>
            <Setter Property="Background" Value="{ThemeResource SystemControlBackgroundAltMediumLowBrush}"/>
            <Setter Property="BorderBrush" Value="{ThemeResource SystemControlForegroundBaseMediumLowBrush}"/>
            <Setter Property="BorderThickness" Value="{ThemeResource ComboBoxBorderThemeThickness}"/>
            <Setter Property="TabNavigation" Value="Once"/>
            <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Disabled"/>
            <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
            <Setter Property="ScrollViewer.HorizontalScrollMode" Value="Disabled"/>
            <Setter Property="ScrollViewer.VerticalScrollMode" Value="Auto"/>
            <Setter Property="ScrollViewer.IsVerticalRailEnabled" Value="True"/>
            <Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False"/>
            <Setter Property="ScrollViewer.BringIntoViewOnFocusChange" Value="True"/>
            <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
            <Setter Property="HorizontalAlignment" Value="Left"/>
            <Setter Property="VerticalAlignment" Value="Top"/>
            <Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
            <Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}"/>
            <Setter Property="ItemsPanel">
                <Setter.Value>
                    <ItemsPanelTemplate>
                        <CarouselPanel/>
                    </ItemsPanelTemplate>
                </Setter.Value>
            </Setter>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="ComboBox">
                        <Grid>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="*"/>
                                <ColumnDefinition Width="32"/>
                            </Grid.ColumnDefinitions>
                            <Grid.RowDefinitions>
                                <RowDefinition Height="Auto"/>
                                <RowDefinition Height="*"/>
                            </Grid.RowDefinitions>
                            <VisualStateManager.VisualStateGroups>
                                <VisualStateGroup x:Name="CommonStates">
                                    <VisualState x:Name="Normal"/>
                                    <VisualState x:Name="PointerOver">
                                        <Storyboard>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Background">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlPageBackgroundAltMediumBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Background">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightBaseMediumBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="Pressed">
                                        <Storyboard>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Background">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlBackgroundListMediumBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Background">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightBaseMediumLowBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="Disabled">
                                        <Storyboard>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Background">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlBackgroundBaseLowBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="Background">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="HeaderContentPresenter">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseMediumLowBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseMediumLowBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="DropDownGlyph">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseMediumLowBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                </VisualStateGroup>
                                <VisualStateGroup x:Name="FocusStates">
                                    <VisualState x:Name="Focused">
                                        <Storyboard>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="HighlightBackground">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightTransparentBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="HighlightBackground"/>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="DropDownGlyph">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseMediumHighBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="FocusedPressed">
                                        <Storyboard>
                                            <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="HighlightBackground"/>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="DropDownGlyph">
                                                <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseMediumHighBrush}"/>
                                            </ObjectAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="Unfocused"/>
                                    <VisualState x:Name="PointerFocused"/>
                                    <VisualState x:Name="FocusedDropDown">
                                        <Storyboard>
                                            <ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetProperty="Visibility" Storyboard.TargetName="PopupBorder">
                                                <DiscreteObjectKeyFrame KeyTime="0">
                                                    <DiscreteObjectKeyFrame.Value>
                                                        <Visibility>Visible</Visibility>
                                                    </DiscreteObjectKeyFrame.Value>
                                                </DiscreteObjectKeyFrame>
                                            </ObjectAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                </VisualStateGroup>
                                <VisualStateGroup x:Name="DropDownStates">
                                    <VisualState x:Name="Opened">
                                        <Storyboard>
                                            <SplitOpenThemeAnimation ClosedTargetName="ContentPresenter" OffsetFromCenter="{Binding TemplateSettings.DropDownOffset, RelativeSource={RelativeSource Mode=TemplatedParent}}" OpenedTargetName="PopupBorder" OpenedLength="{Binding TemplateSettings.DropDownOpenedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}"/>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="Closed">
                                        <Storyboard>
                                            <SplitCloseThemeAnimation ClosedTargetName="ContentPresenter" OffsetFromCenter="{Binding TemplateSettings.DropDownOffset, RelativeSource={RelativeSource Mode=TemplatedParent}}" OpenedTargetName="PopupBorder" OpenedLength="{Binding TemplateSettings.DropDownOpenedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}"/>
                                        </Storyboard>
                                    </VisualState>
                                </VisualStateGroup>
                            </VisualStateManager.VisualStateGroups>
                            <ContentPresenter x:Name="HeaderContentPresenter" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" FontWeight="{ThemeResource ComboBoxHeaderThemeFontWeight}" FlowDirection="{TemplateBinding FlowDirection}" Margin="{ThemeResource ComboBoxHeaderThemeMargin}" Visibility="Collapsed" x:DeferLoadStrategy="Lazy"/>
                            <Border x:Name="Background" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" Grid.Row="1"/>
                            <Border x:Name="HighlightBackground" BorderBrush="{ThemeResource SystemControlHighlightBaseMediumLowBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{ThemeResource SystemControlHighlightListAccentLowBrush}" Grid.ColumnSpan="2" Opacity="0" Grid.Row="1"/>
                            <ContentPresenter x:Name="ContentPresenter" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" Grid.Row="1" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                                <TextBlock x:Name="PlaceholderTextBlock" Foreground="{StaticResource MyCustomPlaceholderForegroundColor}" Text="{TemplateBinding PlaceholderText}"/>
                            </ContentPresenter>
                            <FontIcon x:Name="DropDownGlyph" AutomationProperties.AccessibilityView="Raw" Grid.Column="1" Foreground="{ThemeResource SystemControlForegroundBaseMediumHighBrush}" FontSize="12" FontFamily="{ThemeResource SymbolThemeFontFamily}" Glyph="&#xE0E5;" HorizontalAlignment="Right" IsHitTestVisible="False" Margin="0,10,10,10" Grid.Row="1" VerticalAlignment="Center"/>
                            <Popup x:Name="Popup">
                                <Border x:Name="PopupBorder" BorderBrush="{ThemeResource SystemControlForegroundChromeHighBrush}" BorderThickness="{ThemeResource ComboBoxDropdownBorderThickness}" Background="{ThemeResource SystemControlBackgroundChromeMediumLowBrush}" HorizontalAlignment="Stretch" Margin="0,-1,0,-1">
                                    <ScrollViewer x:Name="ScrollViewer" AutomationProperties.AccessibilityView="Raw" BringIntoViewOnFocusChange="{TemplateBinding ScrollViewer.BringIntoViewOnFocusChange}" Foreground="{ThemeResource SystemControlForegroundBaseHighBrush}" HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" MinWidth="{Binding TemplateSettings.DropDownContentMinWidth, RelativeSource={RelativeSource Mode=TemplatedParent}}" VerticalSnapPointsType="OptionalSingle" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" VerticalSnapPointsAlignment="Near" ZoomMode="Disabled">
                                        <ItemsPresenter Margin="{ThemeResource ComboBoxDropdownContentMargin}"/>
                                    </ScrollViewer>
                                </Border>
                            </Popup>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Application.Resources>
</Application>