You can drag any shape onto the canvas and place it anywhere.  It's not constrained by page margins.  This means it can be placed at the edge of a page or behind text.

The position of a shape is locked to a page.  This means that if you re-paginate move text to another page, the shape will stay on the previous page.

Shapes can be re-sized using the grab handles.  If you want to re-size and change the ratio - hold down the 'Shift' key on your keyboard.

To change the color of the shape use the background tool in the left panel.

You also use this tool to insert a background image into the shape.

More advanced shapes.  
Modern styling uses more organic shapes like waves and blobs.  These can be added in the same way by dragging the elements onto the canvas.

For these elements you can adjust the frequency and depth by moving the sliders, or click re-generate to produce another one.

Layers
If you want a shape to be in front or behind another shape, use the 'Send to back' or 'Send to front' icons on their respective toolbars.

You can also adjust layers by right-clicking on a shape and choosing the layer controls in the popup menu:



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