In order to change the selected state color of EventContainers and SummaryContainers we need to alter their control templates and set the desired color for their selected state representation. Then we should change the EventContaierStyle and the SummaryContainerStyle to set the Template of these containers to the changed ones. Information about styling and theming the RadGanttView can be found here
Please find attached a sample project that changes the SummaryContainerTemplate (<Path x:Name="RightMarkerSelected" ..>, <Path x:Name="LeftMarkerSelected" ... > and <Border x:Name="BackgroundBorderSelected" ... > ) and EventContainerTemplate (<Rectangle x:Name="CommonStatesVisual" .. >) and changes the selected color to Red. Note that the project uses Implicit Styles
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