Only way is to update the Preferences on a role for admins; Users can update their own preferences as well to change the color theme. But the point is it cannot be controlled by admins on a global basis- The main requirement behind this request is to make it very clear for the user which environment they are in. And right now, this cannot be done in a quick way - only custom roles can be changed & admin/full access users need to update their color themes if needed. Also, important thing to note is that color themes will be overwritten each time the sandbox account is refreshed. So the exercise of updating custom roles and asking admin/ full access users to update their color themes needs to happen again and again with every sandbox refresh. One more example how NetSuite does not make it easy to do simple things.