Style is the general layout of the slides. There's options designed accordingly for different presenting purposes. Here's how to locate them.
On the Design editor, click on the toolbar at the right edge of screen. It will display design 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.