property Items.ItemBold(Item as HITEM) as Boolean

Retrieves or sets a value that indicates whether the item should appear in bold.

TypeDescription
Item as HITEM A long expression that indicates the handle of the item.
Boolean A boolean expression that indicates whether the item should appear in bold.

To change the bold attribute for a cell you can call CellBold property. 

The following sample bolds the selected item:

Dim hOldBold As HITEM
Private Sub ExplorerTree1_SelectionChanged(ByVal Group As EXPLORERTREELibCtl.IGroup)
    If (Group.Index = 0) Then
        With Group.Items
            If Not hOldItem = 0 Then
                .ItemBold(hOldItem) = False
            End If
            hOldItem = .SelectedItem()
            .ItemBold(hOldItem) = True
        End With
    End If
End Sub