Thank you for contacting us and for your feedback.
If I understand you correctly, your page has specified either a background color or an image - this gives you ~1 fill rate. Next, when you open the picker's popup, it has a background color, which gives you additional ~1 fill rate. Finally, you have ~1 fill rate coming from the items control which hosts picker's items.
Unfortunately there is not much we can do in this case. You may try some additional workaround - remove the page's background when the control is opened in full screen and restore it back when the control's popup is closed. You may hook the StateChanged event of the control, which will tell you when the control is Expanding, Expanded, Collapsing and Collapsed.
As for the background of the popup part - this is definitely a good addition and we will implement a PopupStyle property which will control the visual appearance of the Popup part when in full screen. I would like to thank you for your feature request - I have updated your Telerik points for your time.
Do not hesitate to write us back should you have any question/problem with our tools.
the Telerik team
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