If you need to style content or amend layouts on a single page, custom CSS code is best entered in the Custom CSS box that can be found under the page inspector, pictured below.
To obtain selector names, it is recommended you use Safari or Firefox to preview your website. When you have developer tools enabled, right-clicking (or Ctrl and Clicking) in the page will open a small menu. Under this menu you will often find an option to “inspect” an element. Clicking this will open a small inspector window which will list selector names, e.g., “nav” or “footer” or “aside”.
You can use standard CSS code in this theme. Detailed information on using CSS can be obtained from the internet.