Shape
Shape provides distinction to individual components as well as establishes a sense of overall visual identity throughout the application.
Border radius
Border radius is used to set the rounded-ness of a container or shape. Consistent use of a container's border radius can be used to clarify intent such as whether or not a component is interactive.
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Circle
Border width
Border width is used to define the edge of a frame or component. This can be used alone or in conjunction with background fills to create various levels of visual hierarchy within a series of containers.