This is how I've managed to do it: Go to layout mode and create a text object (start typing). Next, select that and use the formatting bucket to apply its style to your pop-up menu. Weird but it works...
Great, thanks! A pain to have to do it this way, but I guess we get used to 'workarounds' in FileMaker...
We had the same problem and have posted elsewhere. In case this helps, we believe our problems were down to the defauts set in the converted file, where it appears you cannot obtain all the new formatting tools now available in FMP12. Also, in some cases, you could not reset legacy objects to new formatting styles. This problem appears to be different between converted files, which is why some people report problems and others do not.
For converted files, we're now running a newly created FMP12 file alongside, where we create objects such as pop up fields, stylise as we want them, copy these over to our converted file and use the format painter to apply the same style to our legacy objects.
As you say, 'a pain' but it has worked on all objects so far.
Great tip, Andy. Thanks.