This is, probably, one of most common questions about styling in WPF - how to change selected item background color in ListBox (ComboBox, ListView etc). The answer is really simple - look into default control templates of those controls. Those templates using system brushed in order to paint background colors. So, if you want to customize an appearance of those controls, you always can rewrite templates, but why to do it if the only thing you want is to change color brush of elements, so KISS [keep it stupid simple] rewrite those system brushed like this:
<ComboBox Background="Red">
<ComboBox.Resources>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="Yellow" />
<SolidColorBrush x:Key="{x:Static SystemColors.ControlBrushKey}" Color="Green" />
<SolidColorBrush x:Key="{x:Static SystemColors.WindowBrushKey}" Color="Red" />
</ComboBox.Resources>
<TextBlock>TEST</TextBlock>
<TextBlock>TEST</TextBlock>
<TextBlock>TEST</TextBlock>
<TextBlock>TEST</TextBlock>
<TextBlock>TEST</TextBlock>
<TextBlock>TEST</TextBlock>
<TextBlock>TEST</TextBlock>
</ComboBox>
[결과화면]
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