In the Platform, use the Themes panel on the right to apply a theme to your entire presentation, and the Properties panel to change the style of an individual slide.
Open your presentation in the Platform.
Click the Themes panel on the right.
Browse the available themes and click one to preview it.
Click the theme to apply it to all slides.
Choose and apply a theme from the Themes panel.
Change a single slide's layout
Select the slide you want to change.
Click the Properties panel on the right.
Under Slide style, choose a style from the dropdown.
Change a slide's style from the Properties panel.