Your theme has to be something you can prove in your story - It doesn't have to be a universal truth. This means that your theme doesn't have to be something that happens in real life all the time (providing our logic can accept it, in order for us to believe it).
Whatever story you choose to write, be it a contemporary or a story which requires elements of fantasy such as in horror, science fiction etc- the events of that story have to appear logical.
What is not logical and consequently not believable is:
- A character that has no knowledge of computers and overnight becomes a computer whiz
- A car that goes over a cliff, bursts into flames and the character manages to escape unscratched
These are not believable because they can't and don't happen in real life and our logic doesn't accept them.
Your theme will be believed when you prove it (providing of course you can.) Let's see how you can do that.
We'll start with a theme:
"Hard work leads to success."
Our story is about a character whose goal is to reach a managerial position within the company that he works. For the reader to see how the character will reach his goal I will show him:
- Working hard
- Working long hours
- Using his initiative
- Being responsible
And all those qualities, in the end, will secure him the promotion he has been aiming for.
So my theme here will be proved that ‘Hard work leads to success' because my character succeeds in the end.
From the examples I have given so far, you may have noticed that my stories end on a happy note. Yours don't have to. The ending will depend on the story you are writing and how you, the writer, prefers to end it.
I could have done the reverse with this theme. I could have said,
"Hard work doesn't lead to success."
My story will be the same but in the end I will have the character missing out on the promotion. Both themes will be proved because I have proved them in my story.
Any theme can work in a story providing you can prove it.
Have you proved your theme?