Adding Shapes to your signs can help create containers for elements and text as well as create boundaries for elements. To add a shape, click Shapes in the App Bar, then select a shape to insert it into your slide.

If you look at the template below, shapes are used to create the triangle by setting the background to yellow and the border to white. The bottom of the template also uses the rectangle shape with the same color as the triangle behind the text to help it stand out.

You can customize the style of the Shapes by clicking on the settings icon in the action bar. Through the settings, you can change the border-radius, background colors, add a drop shadow, and more. You can also change the border of the rectangle shape.

By clicking the settings icon, you can change the border thickness and style to apply a border effect. You can even clear the background color to just create a border.
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More information on the Settings panel can be found here, as well as additional info on the Editor here.
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You can add animations to your shapes to have them fly in or fly out, as well as just be animated standing still. To animate your shapes, simply click the shape on your slide, then click the rocket icon. More on animations can be found here.