By default all Lnk.Bio users start with the Dark Mode set to Auto. This means that Lnk.Bio will follow your device settings: if your phone/laptop is set to Dark Mode, Lnk.Bio will also be in Dark Mode, and vice versa.
If instead you wish to fix a specific mode (Dark or Light) always on, independently from your device settings, you can follow these steps:
Head over to your account Settings from Menu > Settings
Scroll down a bit until you see Dark Mode, and click Edit
Select your preferred mode, and click Save
