You can create interesting effects by making objects more or less opaque. When you put a low-opacity object on top of another object, for example, the bottom object shows through.
Select an image, shape, text box, line, arrow, drawing, or video.
If you select a text box, everything in the text box, including the text and any background color, is changed.
For a drawing, drag the Opacity slider in the Format 
sidebar; for any other item, click the Style tab in the sidebar, then drag the Opacity slider.