To switch between dark and light mode within Sleeper on mobile, navigate to the leftmost panel within the app. On the bottom, next to your username, you will see a half-moon icon displayed. Tapping on this icon will toggle between both modes.
Web App: https://sleeper.app/
Currently, in the web browser version of Sleeper, we only offer a dark theme for users. We don't have any plans to offer light mode on the web app at this time.