It's important to stand out from your competition, so you don't want the tropical vacation tour slides you're creating to look like any other company's slides. Changing or customizing a theme can set your presentation apart and make it memorable. Before diving into how to do this, let's spend a moment dissecting themes.
Themes are a combination of theme colors, theme fonts, theme effects, and backgrounds applied to one slide, several slides, or an entire presentation. Themes appear in the Themes gallery on the Design tab. When you open the Themes gallery, you can see which themes are used in the presentation that you have open, custom themes (if any have been created), and the built-in themes.
When you rest your pointer on a thumbnail in the Theme gallery, you get a preview of what the theme would look like if you applied it to your presentation. This is called Live Preview. Without clicking or committing to anything, you can see your slide with dozens of different themes applied to it.