Sometimes one of Google's pre-designed themes is close to what you’d like, but might not be the best color or font for your presentation. Thankfully it’s not too difficult to adjust the theme to suit your purposes.
In Google Slides, each theme has a slide master. Information about the theme, including colors, fonts, and effects, are stored in the master. By modifying the slide master, we can make changes to all of the slides at the same time.