Themes
In the user popover(found in the right-hand corner of the screen), you can find the option to change the theme of the extension. There are currently three base themes: Classic, Harmonic paper and Deep purple. Each theme comes in light and dark mode.
Inside the theme settings, you can enable/disable auto-dark mode, which listens to changes in your system's dark and light mode settings. You can also hide/show the background and the sidebar panels.