Modify PowerPoint theme elements to mix and match colors and fonts. Learn all about using the PowerPoint modify theme feature here.
To further customize your slides, you can change the background color by choosing a different background style. The available background styles will vary depending on the current theme.
If you want even more control over the background, click the Format Background command on the Design tab.
Different themes also include different slide layouts and background graphics. We'll talk about how to customize these in our Slide Master View lesson.