Enter reading page -> Page Display -> Themes. There you can choose from different theme colors. Or you can right click anywhere in the PDF and choose Themes from the pop-up menu.
Light Mode: The default one that displays the document in a bright white background.
Sepia Mode: Displays the document in a light-yellow background, so it’s easier on your eyes.
Dark Mode: Displays the document in inverse-color (like it is on film negatives) in a dark environment.
Reseda Mode: Displays the document in a light-green background to protect your eyes after long-time reading.