This one's going to be a little bit harder to give step-by-step instructions for. To begin with, I spent an embarrassing amount of time on the Star Wars Lego website. If you click on the link and take a look at the home page you'll see that that's where I took the header, side and footer from.
In order to use any of the elements from the website, I had to first save a screen shot containing each piece that I was interested in using. As I said, the main bones of the invite came from the home page, the majority of the rest came from the Comics link. I did remove the navigation bar at the top and elongated the whole thing in order to have room for all of the birthday party details. The picture in the window near the top came from one of the comic episodes, as did the silhouettes of Anakin, Asoka and R2D2 near the bottom.
All of these elements were put together in Gimp (a program similar to Photoshop but
I'm pretty proud of how seamless this looks, especially considering that it consists of at least eighteen different "layers" or elements. While it's hard to explain how this was done -especially to those who aren't familiar with Photoshop or Gimp- I think that this method can be used to make an invitation to fit any theme.
Someone suggested selling customized birthday invites on Etsy or the like, but I'm not sure of the logistics regarding using what I'm assuming are copyrighted images. Think there'd be a market for them?? ;)