Yes! I really need a theme...but thats a lot harder than I expected.
I spent a lot of time on the theme for my build. A couple of things that helped me...
1. Start sketching ideas early. Even if you can't draw, keep a series of sketches and your ideas won't be lost.
2. Goggle an "idea", then select the "Images" tab. I Googled "Classic Arcades", "Dogfights", "Aircraft Art", "80's design"... and a bunch more. As I looked at the images from the search results, it spawned new ideas for my theme.
3. Go to any place that has arcade machines. I went to places that had arcades, and just walking around looking at the games for inspiration.
4. After you think you've got a good theme, revisit it often. I tweaked my theme all the way to the end.
Keep at it.