The PNG files are just preview thumbnails. They aren't the actual menu templates. The menu templates are PSD files -- which may not have been downloaded yet to your computer.
You can do a lot to customize a movie menu right in the movie menu Adjustments workspace in Premiere Elements including repositioning the elements, changing text colors and fonts and replacing backgrounds.
But editing the actual templates in Photoshop is pretty high level stuff. I detail how to do it in my "Muvipix.com Guide to Photoshop Elements & Premiere Elements", available on Amazon -- but the process takes several pages and illustrations to describe. I'd only recommend doing it if you're comfortable with high-level Photoshop editing and layer sets.
Steve - thanks for the response.
It's a bit curious - I spent quite a lot of time changing and saving the PSD files but either got the effect described in my original post or no effect at all.
Yesterday, I looked in more detail at the PNG files and noticed that they were tiny (being thumbnails, as you said) compared with my PNG files, so I made all the relevant layers in my PSD files visible and flattened them to single layers, which I renamed "Layer (0)", having noticed that one of PE's PNG files had a single layer with that name.
I then reloaded PE, brought up the project, changed the menu theme to my amended one and - hey presto! - everything was hunky dory!!!!
To confirm all of this, I repeated the whole process with another menu theme and that worked too. I haven't a clue as to why my new "method" works but have noted down each step so that I don't forget it!
Again, thanks for taking the trouble to respond to my original post.