Set ComboBox selected item highlight color
Override the SystemColors.HighlightBrushKey (and SystemColors.HighlightTextBrushKey if you want):
<ComboBox>
<ComboBox.Resources>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}">Red</SolidColorBrush>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightTextBrushKey}">Blue</SolidColorBrush>
</ComboBox.Resources>
<ComboBoxItem>One</ComboBoxItem>
<ComboBoxItem>Two</ComboBoxItem>
</ComboBox>