view > show master slides
right click on any master > new master slide
add your design elements to be shown on every slide
you can thene add placeholders for text and graphics if prefered here
inspector > slide inspector > appearance
you can now save this as a theme so it can be used on future projects
read the Keynote user manual chapter > designing master slides and themes to see how this is done
thanks for your reply. As far as I understand your suggestion, I would create a master slide the contents of which are then applied to every slide that inherits from that master, then save that as a theme.
However, I think you have misunderstood my aim resp. my problem.
I have a theme with a nicely flexible collection of master slides which I would like to use on several occasions. Let's say there were 20 master slides, and my presentation of let's say 100 slides would make use of a certain set of those.
Now I would set up my my presentation for e.g. ThinkConf 2012, and I would like each slide in my presentation to say "ThinkConf 2012, MyName" next to the slide number. Since the slides would by based on an unforeseeable subset of master slides, I would have to edit every single master slide beforehand. Naturally, I would prefer to take a pass on that work step.
Thanks again, and please reiterate if have misunderstood your suggestion.
Any further help is very welcome!
I conclude that Keynote does not afford this basic function.
If you want a theme to contain certain content on every master slide, you will have to edit every single master slide manually.
This has been faulted at least since 2006 (https://discussions.apple.com/message/2016843#2016843).