<Style TargetType="{x:Type ComboBoxItem}">
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FF3A60AD" Offset="0.528"/>
<GradientStop Color="#FF6A85D8" Offset="0.01"/>
<GradientStop Color="#FF3464C4" Offset="1"/>
<GradientStop Color="#FF202E7E" Offset="1"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
</Style>
You can add a style to target All comboBox Item, which is what is given in my example. Alternatively, you can individually brush each item by modifying each items individual properties.