So that you can see how all your content is going to look, choose a theme for the presentation right at the start. That way you'll see how the theme colors look on certain things you add, such as charts or tables.
A theme supplies the look and feel, in terms of slide design, of the presentation. It applies the background design, placeholder layout, colors, and font styles to your slides and slide elements.
The Design tab is the place to go for themes. A gallery like the one for design templates that you may have used in earlier PowerPoint versions appears here under Themes. Each theme has a name, which shows in the ScreenTip.
When you rest the pointer over a theme thumbnail, the theme is shown in a temporary preview on the current slide. You see the theme's effect before you apply it, saving you the step of undoing it if you don't like it. When you move the pointer off the thumbnail, the preview ends.
Click the arrow on the right of the Themes group to get more choices and information, such as which themes (since you can use more than one) are used in the presentation, and for links to other themes — on Office Online, for example.
The same themes are supported in Microsoft Office Word 2007 and Microsoft Office Excel 2007.