fluffy sylveon

Go into your settings (which can be found in the dropdown menu at the top right, or the hamburger menu at the top left if on mobile), then go to Display, and click the "Theme" dropdown.(Or, as a shortcut if you happen to be using the dark theme, just go to the other domain ( trixiebooru.org if you're using derpibooru.org or vice versa); you'll probably be logged out and so will be shown the light theme instead.)