Major themes
The Unity of Nature, The Danger of Fear, Dreaming (sleeping)
Literally devices
There is the whole extended metaphor of Santiago searching for treasure and finding that treasure to actually be finding love. There are many Biblical allusions. Santiago can be seen as a Christ-figure. Santiago is literally a shepherd while Christ was the figurative "Shepherd of men" . Also consider The character of Melchizedek (the King of Salem) as sort of a God-figure. Santiago has to give his sheep to him. That's akin to Biblical scripture demanding people tithe ten percent of their earnings to God (or the church).