When closed vector network layers such as rectangles (shortcut: R) and circles (shortcut: O) are created, you can use the Fill function to enrich their appearance. In the “Fill” tab, click ➕ to create a new fill. The first layer fill defaults to a solid color fill of #D8D8D8, and subsequent layers default to a solid color fill of #D8D8D8. So you can create various looks by creating multiple layers, using different levels of opacity, and blending layer fill types. Four types of layer fills are currently supported: solid color, linear, radial, and image fill.
i.e., solid color fill, supports setting solid color and transparency.
The shape, direction, and area of the linear gradient can be edited on the artboard or in the color palette.
The shape, direction, and area of the radial gradient can be edited on the artboard or in the color palette.
After switching to the Image Fill field, just select Image Fill at the bottom.