In reference to this other postpost, the ControlBrushKey did not work for me. The InactiveSelectionHighlightBrushKey did, however. Also setting the InactiveSelectionHighlightTextBrushKey got around the changing text color.
My complete solution:
<TreeView.Resources> <!-- Style the inactive selection the same as active --> <SolidColorBrush x:Key="{x:Static SystemColors.InactiveSelectionHighlightBrushKey}" Color="{DynamicResource {x:Static SystemColors.HighlightColorKey}}" /> <SolidColorBrush x:Key="{x:Static SystemColors.InactiveSelectionHighlightTextBrushKey}" Color="{DynamicResource {x:Static SystemColors.HighlightTextColorKey}}"/> </TreeView.Resources>