Add a border to a slide in Keynote on Mac

To add a border — such as a solid or dotted line — around a slide, you first add a square shape to the slide, then change the shape’s attributes so it behaves like a border. For example, you change the size of the shape, give it no fill (as opposed to a colour fill) so that it doesn’t obscure other objects on the slide, and so on.

If you want several slides to use the same border, create a master slide that includes the border.

Click in the toolbar, then select the square or rounded rectangle from the Basic category.
Drag the white squares that appear around the shape until the outer edges of the shape form the border size you want.
In the Format sidebar, click the Style tab.
To remove the colour from the shape or choose a different colour, click the pop-up menu below Fill, then click No Fill or choose a different fill option.
Click the disclosure triangle next to Border, then click the pop-up menu and choose a type of border (line or picture frame).
To change the transparency, drag the Opacity slider.
To lock the border so it doesn’t get moved accidentally, click the Arrange tab at the top of the sidebar, then click Lock.

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Keynote User Guide for Mac

Add a border to a slide in Keynote on Mac