In PowerPoint, any Theme brings to the table several unique facts such as Theme Fonts, Theme Colors, and Theme Effects, and also Slide Layouts, Backgrounds, etc. When you apply another Theme to your PowerPoint presentation, the changes reflect in all slides in the presentation. Yet, the change is complete, and thus the resulting slides all look consistent.
To learn more, choose your version of PowerPoint. If we do not have a tutorial for your version of PowerPoint, explore the version closest to the one you use.