Currently, to achieve this you would need a new skin, or atleast changing the sprite of the combobox. The latter being a much easier task. If you are having troubles modifying the sprite your self, I could assist you on that.
I should note that we are improving the skins in terms of unified look e.g. all textboxes to look the same, all buttons to look the same and so on. As this is a long task and eventually a breaking one in terms of appearance, those improvements will be made available with the official Q3 release.
All the best,
the Telerik team
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