To switch between dark and light mode within Sleeper on mobile, navigate to the leftmost panel within the app. Tap on your username at the bottom, scroll down to "System Settings", the first option will toggle between both dark and light modes as shown below.
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.