View > Display > Outline
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Views toolbar
Toggles on and off the sharper display of object edges in the model. Outline can work in conjunction with Wireframe or any of the shading options (Full Shading, Phong Shading, and Reflective Phong Shading). The example below shows an Outline and Full Shading display.
The default outline color is white on dark backgrounds and black on light backgrounds. Outline automatically switches between white and black based on the current combined color intensity of the background color. With a standard black background, you see white lines. However, if you change to a white or light background, you see black lines suitable for printing.
You can specify the thickness and color of the outline using the Outline tab on the View Settings dialog box. For more information, see Outline Tab (View Settings Dialog Box).