When I first started writing, I despised the idea of a "theme". Even more so when I was given a writing assignment with a pre-determined theme. I wanted to write what I wanted and however I wanted. That attitude seems incredibly childish to me now. Out of my list of current short stories, they are all for different anthologies running off a single theme. Instead of seeing this as a barrier as I once did, I now see it as an exciting challenge or test.
These tests are especially good for my characters. I can see how Esther or Bryan react to specific situations. For example, I am currently writing a short story regarding the theme of failure. So what exactly would happen to cause Esther to fail? And what are the repercussions? Looking over the publisher's specification, I also see they do not want any European Faeries. So Esther fails against a spirit of Asian origins. It isn't too restrictive, but I have certain points that must be established.
Having this vague outline in the beginning can be a lot more helpful in the long run. While I still enjoy sitting down and just writing whatever comes to my mind, those stories tend to take longer to
finish. They usually need a lot of trimming as well in order to cut out my own ramblings.
Of course, these anthologies can be a lot harder to actually get published in. Understandably, the publisher wants to make sure all the pieces are cohesive and fit with their own idea of what the theme means. My example of failure might not be what they are looking for. Or maybe it is. Either way, I've written something I'm proud of.