From Xojo Documentation
A Rectangle control draws a rectangle that can be of any length, width, and background color. By default, rectangles are 150 pixels in length and width, with a black border that is 1 pixel thick and a white center. You can change some elements of the appearance of a Rectangle by via the Style property.
For example, via the Style property you can change the color and thickness of the border.
Left, Top, Width, Height - These values change the position and size of the rectangle.
Style - Provides access to properties that can be used to change the WebRectangle's appearance.
Generally you configure a Rectangle in the Inspector. However you can also change Rectangle properties, such as its Left, Top, Width, Height and Style in code.
For example, in a WebRectangle's Shown event, you can change the border color and thickness like this: