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You can edit the background color as per your choice from
Menu > Preferences > Color Paletter or
File > Preferences > Background
You can customize, save the CDF file which can be used to override the default color palette of NX.
Default 'ugcolor.cdf' can be located in the UGII folder.
I recommend the usage of Visualization Templates. Because it could also be that you do not only want to change the palette, but also other viusalization settings.