Timeless Design means something else: it doesn’t mean that a design looks nice even today, but it is used to describe designs that go beyond shorter lived trends. E.g. imagine modern furniture from the time around 1900 that looks as if it could have been designed today, in the 80s or in 20 years time.

Windows XP doesn’t has a bad design, but it is almost certainly not timeless, alone for the color choice.