Page borders in 'Pages'

Fancy, or just ordinary borders? In the Pages view menu, select show layout. This will show the page boundaries for the current margin settings. Insert a rectangular shape, and set its color fill to white from the color well on the Styles panel. Then set a Border to Line, a color, and a line thickness — expanding it from its thumbnail size to eclipse the main document borders.

While this rectangular shape is still selected, visit the Arrange menu > Section Layouts > Move Object to Section Layout. This is Pages v11.* speak for move the object to Section Master from older Pages versions. The result of this will to create a Pages border in the background which is below the document text layer. Now, you revisit the View menu and select Hide Layout.

Or, you can get fancy by copying and pasting one of the available medium PNG page borders from Openclipart, into the Pages document per the above, rescale it to fit the layout borders, and the previous paragraph above: