You can change the appearance of the site boundary from the boundary toolbar.
Enter boundary edit mode (right click -> edit boundary OR more tools>edit boundary)
Click the Paint Palette icon in the boundary toolbar
Edit the style:
Fill Color: Fill the boundary polygon with a color
Fill opacity: between 0 and 100%. (Hint: make the opacity greater than 0% for the fill to show)
Line color: Select a color from the picker, or double click a swatch to create a custom color
Line opacity: between 0 and 100%. (Hint: make the opacity 0% to hide the boundary outline)
Line width: enter a number to change the line width in pixels
Line style: choose from a selection of dashes or solid
Click Save to commit your Changes.
Click Reset to reset to Giraffe defaults.