You can't edit the button style through the Presenter menu, but once it's on a slide, it's just a PowerPoint object and can be style that way. Right-click on a button and choose to edit the shape. The options aren't great, but will allow you to tweak color and shape properties.
Truthfully, if you are doing a lot of quizzing, you might want to look at Captivate 5. They added styles to Captivate which make it easy to add graphics/styles to all buttons in the project.
I will be out of the office starting 17/12/2010 and will not return until
I will be out of the office on annual leave.
During this time I will have no access to my emails. If your matter is
urgent please see details of who to contact in my absence.
20-22 December: Sonia Savage or Carmen Shields
22-24 December: Sonia Savage
29 December: Carmen Shields
30 December: Sonia Savage
31 December: I will be contactable on 07826921970