WPF Change Background color of a Combobox

If you are using VS 2012 or VS 2013 in the Document Outline window while viewing your xaml get the control template of a combobox. Find the style x:Key="ComboBoxReadonlyToggleButton" and update the RenderMouseOver = "False" and RenderPressed="False" on the Themes:ButtonChrome and also comment out the trigger that affects IsEnabled.

Below is the one on I edited on my box to illustrate what I mean.

<Style x:Key="ComboBoxReadonlyToggleButton" TargetType="{x:Type ToggleButton}">
        <Setter Property="OverridesDefaultStyle" Value="true"/>
        <Setter Property="IsTabStop" Value="false"/>
        <Setter Property="Focusable" Value="false"/>
        <Setter Property="ClickMode" Value="Press"/>
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ToggleButton}">
                    <Themes:ButtonChrome x:Name="Chrome" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" RenderMouseOver="False" RenderPressed="False" SnapsToDevicePixels="true">
                        <Grid HorizontalAlignment="Right" Width="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}">
                            <Path x:Name="Arrow" Data="{StaticResource DownArrowGeometry}" Fill="Black" HorizontalAlignment="Center" Margin="3,1,0,0" VerticalAlignment="Center"/>
                        </Grid>
                    </Themes:ButtonChrome>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsChecked" Value="true">
                            <Setter Property="RenderPressed" TargetName="Chrome" Value="False"/>
                        </Trigger>
                        <Trigger Property="IsEnabled" Value="false">
                            <!--<Setter Property="Fill" TargetName="Arrow" Value="#AFAFAF"/>-->
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

Try this

 <Window.Resources>  //Put this resourse n Window.Resources or UserControl.Resources
   <LinearGradientBrush x:Key="NormalBrush" StartPoint="0,0" EndPoint="0,1">
      <GradientBrush.GradientStops>
         <GradientStopCollection>
            <GradientStop Color="#FFDC3939" Offset="0.0"/>
            <GradientStop Color="#FFE80E0E" Offset="1.0"/>
         </GradientStopCollection>
      </GradientBrush.GradientStops>
   </LinearGradientBrush>

    <SolidColorBrush x:Key="WindowBackgroundBrush" Color="#FFFBE618" />

    <ControlTemplate x:Key="ComboBoxToggleButton" TargetType="ToggleButton">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition />
                <ColumnDefinition Width="20" />
            </Grid.ColumnDefinitions>
            <Border x:Name="Border" Grid.ColumnSpan="2" CornerRadius="2"
  Background="{StaticResource NormalBrush}"
  BorderThickness="1" />
            <Border 
  Grid.Column="0"
  CornerRadius="2,0,0,2" 
  Margin="1" 
  Background="{StaticResource WindowBackgroundBrush}" 
  BorderThickness="0,0,1,0" />
            <Path 
  x:Name="Arrow"
  Grid.Column="1"     
  HorizontalAlignment="Center"
  VerticalAlignment="Center"
  Data="M 0 0 L 4 4 L 8 0 Z"/>
        </Grid>
    </ControlTemplate>

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


<Style x:Key="{x:Type ComboBox}" TargetType="ComboBox">
  <Setter Property="Template">
   <Setter.Value>
     <ControlTemplate TargetType="ComboBox">
      <Grid>
       <ToggleButton 
         Name="ToggleButton" 
         Template="{StaticResource ComboBoxToggleButton}" 
         Grid.Column="2" 
         Focusable="false"
         IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
         ClickMode="Press">
      </ToggleButton>
      <ContentPresenter
        Name="ContentSite"
        IsHitTestVisible="False" 
        Margin="3,3,23,3"
        VerticalAlignment="Center"
        HorizontalAlignment="Left" />
       <TextBox x:Name="PART_EditableTextBox"
         Style="{x:Null}" 
         Template="{StaticResource ComboBoxTextBox}" 
         HorizontalAlignment="Left" 
         VerticalAlignment="Center" 
         Margin="3,3,23,3"
         Focusable="True" 
         Background="Transparent"
         Visibility="Hidden"
         IsReadOnly="{TemplateBinding IsReadOnly}"/>
      <Popup 
        Name="Popup"
        Placement="Bottom"
        IsOpen="{TemplateBinding IsDropDownOpen}"
        AllowsTransparency="True" 
        Focusable="False"
        PopupAnimation="Slide">
        <Grid 
          Name="DropDown"
          SnapsToDevicePixels="True"                
          MinWidth="{TemplateBinding ActualWidth}"
          MaxHeight="{TemplateBinding MaxDropDownHeight}">
           <Border 
            x:Name="DropDownBorder"
            Background="{StaticResource WindowBackgroundBrush}"
            BorderThickness="1"/>
           <ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True">
            <StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
           </ScrollViewer>
          </Grid>
         </Popup>
        </Grid>
       </ControlTemplate>
      </Setter.Value>
     </Setter>
    <Style.Triggers>
   </Style.Triggers>
  </Style>
 </Window.Resources>
 <Grid>
    <ComboBox HorizontalAlignment="Left" Margin="256,57,0,0" VerticalAlignment="Top" Width="120">
    </ComboBox>

 </Grid>

Here is the complete style that you can change : http://msdn.microsoft.com/en-us/library/ms752094%28v=VS.85%29.aspx

While I like the accepted answer the problem I have with it is the text box presenter was completely missed and thus you can select and stylize things but they will never be presented to the end user. I think the easiest way to achieve a combo-box that shows data and then when chosen displays it would be this:

<Window 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:local="clr-namespace:Test" x:Class="MainWindow"
        mc:Ignorable="d" Title="MainWindow" Height="100" Width="200">
  <Window.Resources>
    <Style x:Key="ComboBoxTest2" TargetType="{x:Type ComboBox}">
      <Setter Property="Template">
        <Setter.Value>
          <ControlTemplate TargetType="ComboBox">
            <Grid>
              <ToggleButton Grid.Column="2" Focusable="false" IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}" >
                <ToggleButton.Template>
                  <ControlTemplate>
                    <Grid>
                      <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="5*" />
                        <ColumnDefinition Width="*" />
                      </Grid.ColumnDefinitions>
                      <Border x:Name="Border"  Grid.ColumnSpan="2" CornerRadius="5" Background="Beige" BorderBrush="Black" BorderThickness="1" />
                      <Border Grid.Column="0" CornerRadius="5,0,0,5"  Margin="1"  Background="AliceBlue"  BorderBrush="Black" BorderThickness="0,0,1,0" />
                      <Path x:Name="Arrow" Grid.Column="1"  Fill="Orange" HorizontalAlignment="Center" VerticalAlignment="Center" Data="M 0 0 L 4 4 L 8 0 Z"/>
                    </Grid>
                    <ControlTemplate.Triggers>
                      <Trigger Property="ToggleButton.IsMouseOver" Value="true">
                        <Setter TargetName="Border" Property="Background" Value="Green" />
                      </Trigger>
                      <Trigger Property="ToggleButton.IsChecked" Value="true">
                        <Setter TargetName="Border" Property="Background" Value="Green" />
                      </Trigger>
                    </ControlTemplate.Triggers>
                  </ControlTemplate>
                </ToggleButton.Template>
              </ToggleButton>
              <ContentPresenter Name="ContentSite" IsHitTestVisible="False"  Content="{TemplateBinding SelectionBoxItem}" ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Margin="3"  />
              <TextBox x:Name="PART_EditableTextBox" Visibility="Hidden" IsReadOnly="{TemplateBinding IsReadOnly}"/>
              <Popup Name="Popup" Placement="Bottom" IsOpen="{TemplateBinding IsDropDownOpen}" AllowsTransparency="True"  Focusable="False" PopupAnimation="Slide">
                <Grid  Name="DropDown" SnapsToDevicePixels="True" MinWidth="{TemplateBinding ActualWidth}" MaxHeight="{TemplateBinding MaxDropDownHeight}">
                  <Border x:Name="DropDownBorder" Background="Blue" />
                  <ScrollViewer SnapsToDevicePixels="True">
                    <StackPanel IsItemsHost="True" />
                  </ScrollViewer>
                </Grid>
              </Popup>
            </Grid>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
      <Style.Triggers>
      </Style.Triggers>
    </Style>
  </Window.Resources>
  <StackPanel Margin="10">
    <ComboBox VerticalAlignment="Top" Width="120" Style="{StaticResource ComboBoxTest2}">
      <ComboBoxItem>Item1</ComboBoxItem>
      <ComboBoxItem>Item2</ComboBoxItem>
    </ComboBox>
  </StackPanel>
</Window>

The key parts of the presentation needed to be done is the template 'ToggleButton.Template' for the initial display. I chose to ignore making more brushes and links to more templates and just do it all inline so others could play with it as needed. I also chose what I thought were easily Identifiable colors to just jump to for reference it should look like below when it works: