I found a Workaround with Code behind, so I'll share it with you:
C#:
private void TabControl_SelectionChanged(sender As Object, e As SelectionChangedEventArgs)
{
for(int i = 1; i < 4; i++)
{
FindName("Date" + i).Content = null;
FindName("QuitButton" + i).Content = null;
FindName("SystemBorder" + i).Content = null;
}
var tc = (TabControl) sender;
var index = tc.SelectedIndex + 1;
FindName("Date" + index).Content = FindResource("DateBorder");
FindName("QuitButton" + index).Content = FindResource("QuitButton");
FindName("SystemBorder" + index).Content = FindResource("SystemBorder");
}
VB:
Private Sub TabControl_SelectionChanged(sender As Object, e As SelectionChangedEventArgs)
For i = 1 To 3
FindName("Date" & i).Content = Nothing
FindName("QuitButton" & i).Content = Nothing
FindName("SystemBorder" & i).Content = Nothing
Next
Dim tc = CType(sender, TabControl)
Dim index = tc.SelectedIndex + 1
FindName("Date" & index).Content = FindResource("DateBorder")
FindName("QuitButton" & index).Content = FindResource("QuitButton")
FindName("SystemBorder" & index).Content = FindResource("SystemBorder")
End Sub