NGUI: How to remove black outline on button?

When I try to create button for my game, black outlines appear.

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The circle at the top is just an empty Game Object with Sprite Renderer. At the bottom is the button made with NGUI.
Atlas’ shader is Unlit/Transparent Colored.

In square sprite it can be often fixed by adding the border. The only thing I can think of that can help in your case it switching Trim Alpha off in the Atlas maker.