You can insert shapes (rectangles, circles, triangles) into your prezi by selecting 'Insert' and then 'Symbols& shapes...' from the top menu.
You can select from the different categories that appear or search for a specific symbol or shape using the search bar. Note: You have to select a category before the search bar will appear.
After you have selected the shape that you want to add to your prezi, double- click it and it will appear on your prezi canvas. Alternatively, you can drag it across.
Click your symbol when in Edit Mode to bring up size control points. Use these control points to resize your object. Click 'delete' to remove the object from your canvas.
Double-clicking a shape also bring up the color palette, and thickness controls use these button to change the color and shading of your shape according to the settings of your theme. To customize your shape's color options, select 'Themes' from the top menu. From here you can also edit the color of lines and marks you make with the pencil or highlighter.
Note: Shapes behave like frames, so any object placed within a shape will be grouped with the shape (press "ALT" to remove an object or block of text from the grouping). It is not possible to write text directly onto shapes, but you can create text on a blank part of your prezi canvas and then drag it onto the shape you have made.
More specific changes to the color and border of a shape can be made via the CSS editor.