I'm not really sure how to accomplish this with this theme.
The Filament Group has shown some promise in this demo. But not all that glimmers is gold, since the accordion and other plugins won't work correctly (it's a work in progress).
Related questions include: Round Corners in IE using ui-corner-all
Not possible IE7+ without other CSS/Image techniques
Follow-up:
If you did want to apply some hackery, there is a way to "easily" apply rounded corners in IE - that is, someone has done all the work and you just have to include a few files and change some CSS: http://css3pie.com/
