General:
* Subject
* Topic
* Idea
* Concept
* Motif
* Message
* Issue
* Focus
* Argument
* Line of thought
In literature and art:
* Leitmotif
* Central idea
* Dominant idea
* Underlying message
* Core concept
In music:
* Melody
* Tune
* Motif
In design:
* Concept
* Style
* Mood
* Atmosphere
In everyday language:
* Point
* Idea
* Subject matter
* Key point
* Underlying message
More specific synonyms:
* Recurring theme: pattern, motif, trope
* Central theme: main idea, core message, dominant theme
* Abstract theme: concept, idea, philosophy
* Concrete theme: event, situation, object
The best synonym for "theme" will depend on the specific context in which it is used.