How to tweak the theme.
The theme you choose is a complete design. But where's the fun if you can't do some tweaking? The Design tab has other galleries in case you want to modify the theme.
Each provides a preview on the slide as you rest the mouse pointer over gallery choices.
Colors There's a set of colors for every available theme; you can select your theme's colors and modify any of them.
Fonts Click any sample from the Fonts gallery to apply it to the slides. Each includes a font for title text and one for body text.
Effects These are a range of visual effects for shapes, such as a glow, soft edges, or a three-dimensional (3-D) look.
Background Styles Subtly change the theme's background while keeping within the set of theme colors. Samples are shown in the picture.