In PowerPoint Slide Master view lets you edit all slides at once. Use the Slide Master PowerPoint feature to ensure a consistent look.
When you modify the master or layouts in Slide Master view, you're actually creating a custom version of the current theme. If you want to apply the theme to other presentations, you'll need to save it.
If you're not in Slide Master view, you can save the theme from the Design tab. Just click the drop-down arrow in the Themes group, then select Save Current Theme.