The theme of your story needs to be something you can prove in your story from start to finish. It does not necessarily has to be a realistic details about life but something, that provides logic that we can relate with.
In whatever story you wish to write always remember that even if it is in fiction it needs to have that epitome of logic you yourself can believe in.
So if your story does'nt read logical to the reader then it is unbelievable.
Take for example.
A man who can not swim put swims a river to it's bank.
Jumping high off a 10 story building on landing un-scratch. etc.
These types of logic will not even seem a story come to think of it making any logical sense to someone who as life in them.
Lets start a logical theme here.
"Perfecting your ability to sing well".
This story is a bout a young girl who is aspiring to become a musician in the nearest future.
For your readers to make sense with the story she (character) will need to,
-. Take time out on voice coaching
-.Practice for long hours
-. Use her initiative with are voice balancing ability
-. Take responsibility and decipline as her bench mark to greatness
You can as well tell that all of this quality will guarantee her a future with music.
This will make it provable that the theme "Perfecting your ability to sing" because the character has made head way by the time she is through with each process all the way.
In otherwords i can as well change the feel on my theme to end on sad note so long as i can prove the datails of the story to make a logical sense al the way till the end.