There is no default appearance in WPF Popup. This is just a place for content that needs to pop up.
It’s just that your popup has an element that has a background property, and set it to the desired color, as in this example, from the Popup documentation class .
<Popup Name="myPopup">
<TextBlock Name="myPopupText"
Background="LightBlue"
Foreground="Blue">
Popup Text
</TextBlock>
</Popup>
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