* A specific mustache: Maybe you're thinking of a particular mustache style, like the handlebar or the walrus. The theme of a specific mustache style could be masculinity, power, rebellion, fashion, or even humor.
* The mustache as a symbol: Mustaches can symbolize many things, including age, wisdom, authority, or even humor. The theme of a mustache as a symbol would depend on the context in which it's used.
* The mustache in literature or film: Perhaps you're thinking of a specific work of art that features a prominent mustache. In this case, the theme would be whatever the mustache symbolizes within that particular work.
To help me understand what you're asking, could you please provide more context? For example, what are you thinking about when you say "The theme of the mustache?"
Once I have more information, I can provide a more helpful answer.