The default color for a workspace background on the Wall is black, this is referred to as “Dark Theme”. But you can change the workspace background to white if you would prefer, which is known as “Light Theme”.
To change the background color of a workspace using the Wall client:
Tap the (More) option in the toolbar.
A context menu opens. Tap the Theme option.
A second context menu opens. To change to light mode, tap the Light option.
The screen then updates and displays a white background, as pictured below.
To return to dark mode, tap the Theme option again, then tap the Dark option.
If you want to change your background color on the Web client, read How to Change the Background Color of a Workspace (Web).
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