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In the menu bar, select GitHub Desktop, then click Preferences.  
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Click the Appearance pane.   
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Click the theme you'd like to use. If you want GitHub Desktop to always match the theme on your computer, select System. 
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Use the File menu, then click Options.  
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Click the Appearance pane.   
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Click the theme you'd like to use. If you want GitHub Desktop to always match the theme on your computer, select System.