Rectangle
You can add a new rectangle element by clicking on the Square icon in the toolbar.
Positioning
In this panel group, you can change the position of the rectangle. You can also change the size of the rectangle by dragging the corners of the rectangle.
If you want to resize the element without changing the aspect ratio hold the shift key while dragging the resize handles. If you want to resize it from the center hold the
option (macOS) or alt (Windows) key while dragging the resize handles.
Fill & Stroke
You can select different fill and stroke options. It's possible to use a solid color or a gradient.
If you want to change the opacity, you also do it here.
You can read more about the different fill options in the Fill section.
Corner Radius
You can change the corner radius of the rectangle.
Shadow
You can add a shadow to the rectangle. You can change the color, blur, horizontal offset, vertical offset and opacity. It can be turned on and off by clicking the toggle left of the color picker.