To access all themes settings open Theme Editor (Shopify Admin > Themes > Customize) and click on 'Theme Settings' tab.
The theme comes in 2 styles, to change the current style. Scroll down, click on 'Change theme style' button and select the desired theme style. Note: if you did any changes to the color settings, it will be overwritten.
You can customize the style of your checkout pages in the theme editor (Theme Editor > Settings > Checkout). Add your company logo, change the colors, or choose a new font to make the checkout match your business.