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