Every new presentation starts out with the default theme, called Office Theme. Now see how to choose another one.
To find and apply a theme, click the Design tab on the Ribbon.
Theme samples, in the form of the small thumbnails you see here, are shown in the Themes
To see additional themes, click the More
button on the right of the group.
When you point to any theme thumbnail, a preview of it is shown on the slide.
Click a thumbnail to apply that theme to all your slides. You can also apply the theme only to the slides you have selected.
Apply a theme at any stage of creating the show. However, remember that themes can alter the position of placeholders, so some will work better than others with your content.