Custom FMP12 themes possible
I didn't see this mentioned anywhere yet, so I'll take a stab at the subject despite my relative ignorance. It seems FMP 12.0v1 does accept custom themes. Here's what I tried (sorry, this was done on OS X and I don't know how to translate the instructions for Windows):
1) Go to the FMP12 application folder, select FileMaker Pro, right-click and show package contents, then go to Contents/Resources/Themes. Copy one of those subfolders out of there and into somewhere handy (I copied "Beach" onto my desktop).
2) Go to your FMP support folder and locate the "Themes" folder; I found mine at ~/Library/Application Support/FileMaker/Extensions/Themes. Rename your folder from step 1 to something original (example: Beach2) and drop it into the Themes folder.
3) In manifest.xml, change the <name> parameter to something unique for demonstration's sake; I changed mine to Beach2. Also change the <id> to something unique (I went with com.filemaker.theme.beach2), or else it gets your custom theme confused with the original you duplicated (?). Save.
4) In FMP12, go to change a layout's theme, and you'll find your duplicated theme (Beach2 for me) in the list. Success...? (You'd of course want to customize the css and/or images for there to be any point in having your own custom theme. This is just proof of concept.)
I've been messing around a little bit to make sure that css customizations actually take, and they do. I've also tried applying the theme and then removing my custom theme's folder to make sure the layout retains its theme without the local folder; it does, and you can even still use the "Apply theme styles" to revert objects back to their theme style at this point. I tentatively take this to mean other users won't need a copy of your theme's folder to properly see (or even modify) a layout you created with it; it's apparently only needed when applying a theme.
I really know just enough about this stuff to be dangerous, so I'd appreciate if someone could offer more information about how the manifest.xml's parameters should be structured and how/when the layout makes use of your custom folder. Also instructions for Windows. I'll continue to poke around.