<ComboBox x:Name="comboBox1" Style="{StaticResource ComboBoxBlue}" HorizontalAlignment="Left" Margin="10,128,0,0" VerticalAlignment="Top" Width="75" />

public static List<string> MyItemSource = new List<string>()
{
    "Item 1", "Item 2", "Item 3", "Item 4"
};

comboBox1.ItemsSource = MyItemSource;

<!-- ComboBox Blue Item -->
<Style TargetType="{x:Type ComboBoxItem}">
    <Setter Property="Foreground" Value="White" />
    <Setter Property="Background" Value="Blue" />
    <Setter Property="BorderBrush" Value="Blue" />
</Style>

<Style x:Key="ComboBoxBlueItem" TargetType="{x:Type ComboBoxItem}">

<ComboBox.ItemContainerStyle>
    <Style TargetType="{x:Type ComboBoxItem}">
        <Setter Property="Background" Value="Blue" />
    </Style>
</ComboBox.ItemContainerStyle>