Lumin Home
From the Lumin home page, click your profile icon in the top-right corner.
Then, select Theme.
Select Dark to change to dark mode or select Device to match your device settings.
PDF Editor
Open your document.
Select the three vertical dots to bring up More options.
Then, select Dark mode.
Switch back to Light Mode by clicking the light mode logo.
NOTE: The dark mode feature is currently only available in the editor and is not yet available anywhere else, in the app or on mobile.