I don't think that conditional formatting will work with the active cell. A macro can do what you want if you're interested.
I do know and also that any action performed by a macro cannot be undone. However, to do what you requested requires a macro so you have to decide which way to go.
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
'Modified 3/9/2019 6:47:30 PM EST
ActiveSheet.UsedRange.Interior.ColorIndex = xlNone
Selection.Interior.ColorIndex = 4
End Sub