The RadComboBox uses a css sprite for its rendering. With altering the background style of the Input it only affect the Input area of the ComboBox, which is colored as white in your particular scenario.
The slight gap you had observed is the rest of css sprite, used by the RadComboBox and, as you could well guess, it could not be affected by the
styling of the combo.
Previous versions of the ComboBox, all the way to 2012 Q3, did not have different appearance for read/write and read only combos. However with the aforementioned release, we included improvements in the combobox, which you can see in this detailed forum thread -- http://www.telerik.com/community/forums/aspnet-ajax/combobox/possible-breaking-changes-2012-q3.aspx
In short, we tweaked the styles so it's visible if a combo is read/write or read-only by tweaking the sprite. Our believe is that the improved appearance aids usability.
That said, if you would like to "revert" the looks that's possible with tweaking the sprite. While we do not recommend that, yet you could do it easily. If you encounter any difficulties, we could assist you by providing you with the proper sprite image.
the Telerik team
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