No matter what you write, you will have some kind of theme in mind. Even fiction has themes. Love conquers all, is one example. Good triumphs over evil is another. You may not state it outright in your fiction, but it is there. In fact, it’s better if you don’t say it right out in the work. No one speaks in a thematic fashion.
Sometimes you may not even be aware of your theme when you start, but it underlies your writing. Rest assured of that. In fact, a single work of fiction can have many themes. The Lord of the Rings has several themes. They include friendship, Christ figures, father son relationships, and several others. That’s a long work, so you might not have quite so many, but the themes are present if you look for them.
My Moonbeam mystery has several themes too. Its themes include the truth will come out, friendship can be eternal, in life there are few short cuts. That’s just off the top of my head. My novel, Accidental Colony, has a few themes as well. Friendship, survival, team work, are just a few. When I wrote the novels, I was just interested in telling the story. The themes just crept in without me noticing. I suspect that happens a lot as well. Of course, if you have a theme in mind, you will find yourself shaping the work to fit your theme. That’s fine.
You could start a work with a theme in mind, bring a plot in and add some characters. Poof! You have a story. The point is that you will have at least one theme in your story. Do a scan of your work and you will see the themes. Trust me, they are in there. Your own personal beliefs will color your work. Then again, your readers’ personal beliefs will come out in the work as well. They will see what they see and adapt it to their beliefs. This is not necessarily bad. It just means that different people will get different thoughts from your work.
The point here is that you shouldn’t be surprised if some people see things in your writing that you didn’t realize you had put in there. Be aware of your theme, but don’t preach it. All you have to do is make sure your theme doesn’t overpower the story. That’s part of the preaching. No one wants to read preachy text. If they did, they’d be looking for something to read in the religion section of the bookstore. Your story will be better for it and your readers will appreciate it. If you aren’t sure what the theme is, don’t worry, it’s in there. Just write the story. The theme will emerge.