When I think about theme music, I think of how I can graft a particular song to an emotional mood and then use that song to inspire emotional energy I can use for my magical or business work. Not just any song will do. For instance, I'm listening to DJ Soulslinger while I write this. It's good music, but it doesn't pitch an emotional response in me, so while I enjoy listening to it, I wouldn't use it as a theme. The song has to be something that resonates on an emotional level, something that gets you to feel something, and something that you can sing within your own head, when you can't listen to it. And the emotional response needs to be something you can actually use. If a song makes you feel depressed and you can't find a magical use for it, then it's not a theme song. It might be a song you listen to when you feel down, but a theme song is a call to action, a call to make a change. Now if you can use that song as a way to channel your feelings of depression then it could be a theme song.
Your song doesn't need to have words either. I love the score to Dynasty Tactics 2 and can hum it and find inspiration from it. I've used that score in magical work, as a way to charge my energy. There's no words, but the sounds are enough to invoke an emotional response and that's enough to make it work.
It really comes down to finding music that makes your soul sing and consequently stirs the very power within you necessary to make magic happen. Different music can and does embody different themes. What I would use for a wealth magic working would differ from what I'd use for a love magic working. So long s you can identify a theme that a song fits...use it. You don't need to listen to it to invoke the theme. Just sing it and let it carry your emotions into the magical work you'll be doing.