This property determines how wide a
GuiObject|GUI's border should render, in pixels.
GuiObject/BackgroundTransparency determine how the border of a GUI element should look.
The border width extends outward the perimeter of the rectangle. For instance, a GUI with a width of 100 pixels and BorderSizePixel set to 2 will actually render 102 pixels wide.
Setting this to 0 will disable the border altogether.
This code sample causes the border of a parent GuiObject to highlight when the user hovers their mouse over the element.