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In classic Outlook, select File > Options > General.
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Under Personalize your copy of Microsoft Office, select the drop-down menu next to Office Theme and select a theme.
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Select OK to save your changes.
 
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In new Outlook, in Mail, on the View tab, selectÂ
 Â View settings. - 
                      
Select General > Appearance.
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Select a theme from the list of options. (The effect of the change can be seen immediately in the Settings dialog itself.)
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Select Save.