Change a presentation theme
You can change the theme of a presentation at any time, and all your slides inherit the attributes of the new theme, including object styles, slide backgrounds, and more. If you customized an object from the original theme—if you changed the appearance of title text on a slide, for example—that object retains its changes.
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.