Marvin has the ability to change the text and background colour of pages in your book. Each text/background colour combination is called a theme, and you can configure up to three of them to suit your reading environment (e.g. day mode and night mode). Themes can be set up and selected from the reading menu within the book by tapping the "Aa" menu.
Rather than opening the reading menu each time to select a theme, you can set up a gesture to switch through them quickly.
More details
in the tutorial here.