What is the theme of "If I Forget Thee, Oh Earth..." by Arthur C Clarke?
This is probably Clarke's most famous cautionary tale, which is a story with a warning about what will happen in the future if society continues along its present course. The science fiction genre often embraces this leit motif (recurrent theme), which is indeed one of its principal characteristics.
'Take care of the planet before it's too late' could be one theme, or perhaps 'The armaments race will ultimately lead to an atomic holocaust.' Another could state something like this: 'Mankind will not inherit a future world better than the one he bequeaths today.'