The theme best supported by the excerpt is that life is precious. The story is about a scientist that discovers that his universe has fallen into an equilibrium that will ultimately lead to time to cease to exist, but the very fact that everyone's lives could end at any given moment is what makes them a beautiful miracle.
The scientist remarks: " The buildings we have erected, the art and music and verse we have composed, the very lives we’ve led: None of them could have been predicted, because none of them were inevitable. Our universe might have slid into equilibrium emitting nothing more than a quiet hiss. The fact that it spawned such plenitude is a miracle..."
Life is only beautiful because it can end as suddenly as it began.