Google Slides Themes
Why be like everyone else, edit the Google Slides master to create custom themes!
Notice there is a “Master” slide that rules all of the layouts. Click on that top rectangle to set the font style for the ENTIRE Slides presentation. Change the font of the title style and of the text style.
Try using fonts.google.com to view font choices. You can also use fonts.google.com to find fonts that pair well together. Use one font for the title style and find a good complimentary font for the text style.
The advantage to making changes on the Slide Master is to make global changes. If you decide you would rather use a different font, go to the View menu and choose Master. Change the font on the master and the entire Slides presentation has the updated font.