Change object transparency in Keynote on Mac
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.
Click to select an image, shape, text box, line, arrow, drawing, or video, or select multiple objects.
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.