J O H N  A L L A N wrote:
Magazines will usually have an "editorial calendar", which is usually a years worth of concepts per month. They'll also have their lead time listed, like 2-3 months from publication for submission. So you can plan accordingly.
Some calendars are more specific than others (some will just say something like: August 2014 - fire).
Anyway, you can ask the magazine for theirs.

This.  Take Dark Beauty Magazine for example; when you go the their submission section they have all the details for the themes laid out until December and also how many different looks you need for an editorial. 

Some smaller magazines are more lax though and will take images regardless of the theme as long as they are up to their quality standard and generally adhere to the style of the magazine ie. a fashion magazine isn't going to publish racy glamour most likely.

It depends.  Sometimes I see what people are looking for but if I have a concept I really like I just shoot it and submit to multiple magazines and see who accepts it.  Really even if you adhere to a magazines theme that doesn't guarantee you'll get in so if it's something you don't like it might not be worth the effort.