How to write a good theme

A good theme should be written in the form of a complete sentence.  It should also be a statement for which you can provide textual evidence in the form of quotations from the book.  Further, a theme should be universal in nature, not written in such a way that it is application only to one particular book or character but is, rather, something that applies to people or life more generally.  
For example, you might say, "The individual is always at odds with society," rather than, "This particular character is at odds with his or her society."
You might say, "Nature does not care about human suffering," rather than, "Nature does not care about a particular character's suffering."
Finally, you might say, "It takes a crisis of confidence in order for individuals to grow," rather than, "It takes a crisis of confidence in order for a particular character to grow."