Choose Extensions from the menu -- OR -- Press Cmd+Shift+P or Ctrl+Shift+P and search 'Preferences: Color Theme' then select, '+ Browse Additional Color Themes'
Search for 'Flutter Theme'
Click Install or press Enter on the Flutter light or dark theme.
Press Cmd+Shift+P or Ctrl+Shift+P and search 'Preferences: Color Theme' then select 'Flutter Light' or 'Flutter Dark'