Style is the general layout of the slides. There's options designed accordingly for different presenting purposes. Here's where to locate them.
On the Design editor, display the menu from the toolbar on the right edge of screen. Just hover over it to expand options.
Style is the first design option, click on it to expand. Each Style has different graphic looks for you to choose the best fit to your deck.
Once you choose the Style, use the Back button to return to design menu.
When you chose the right Style, Colors and Fonts, you can save it as a Formula and apply it to future similar decks.