You can change a slide’s theme, master, or layout at any time.
Theme: Choose a different theme when you want to change a slide’s overall look and feel—for example, its background color, fonts, and “tone” (professional or fun). To learn how, see Changing a Slide’s Theme.
Master: Choose a different master slide when you want to use a different predefined layout. For example, you could easily change a slide from a vertical photo layout to a horizontal photo layout by selecting a new master with the layout you want. To learn how, see Applying a New Master to a Slide.
You can also change the layout on a slide’s master—or add new elements to a master, such as a logo—when you want that change to appear on all the slides that use that master. To learn how, see Making the Same Change on Multiple Slides.
Layout: Customize a slide’s layout when you want to add, delete, or modify slide elements such as text boxes and object placeholders without modifying its master or selecting a different one. To learn how, see Customizing an Individual Slide’s Layout.
For more details about slide themes and master slides, see About Themes and Master Slides.