Change a Keynote theme on Mac
You can change the theme of a presentation at any time. Your existing slides inherit the attributes of the new theme, including object styles, title styles, and more. If you customized anything on your slides—for example, if you changed slide backgrounds or the color of text—you can choose to retain those changes.
Note: You can’t change the theme of a presentation you’re sharing with others.
Change the presentation theme
Choose File > Change Theme (from the File menu at the top of your screen), then select a new theme.
Deselect the “Keep your style changes” checkbox if you want the new theme styles to override your style changes.
Some themes aren’t downloaded to your computer until you choose them or open a presentation that uses one. If your connection is slow or you’re offline when this happens, placeholder images and slide backgrounds in the presentation may appear at a lower resolution until you’re online again or the theme finishes downloading.