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A colleague discovered a layout problem when he was using the People Search Box Web Part outside a SharePoint Search Center.
When you add the People Search Box Web Part to your page, the layout looks fine.
The problem arises when you click on the Search Options link.
As you can see, the property box is shown with a transparent background and the labels are almost unreadable.
To solve this problem, you could add the following CSS styles to your custom stylesheet or inside the master page itself.