I was googling for topics related to testing an application that has XP-Themeing enabled and found only a handful of valuable information.
I noted some of them down here (for me) and appreciate any hints from you if you have experience in testing this particular topic, so I can extend the checklist before I start the work on this topic.
test on non-XP operating systems and on XP when themes are not enabled
check all dialogs for controls that may (accidentially) not change its face.
Resize / shrink all dialogs and check its content
4. Tooltips (do they appear and does the text fit into it)
5. Switch between different themes