How do I Reduce the width of the confirmation dialog

[webgeek247]

I am designing a new skin for my forums that is 760px in width and centered, however I am having trouble reducing the width of the confirmation dialog that appears when you post a new thread or reply eg: "Thank you for posting you will be redirected......." the width of that is still set to what it was on the original I used to have where the original filled the entire browser window. Does anyone know how to change this?

Thanks