There is nothing like being unprepared when your presentation venue is incompatible with the slide background you’ve chosen. One solution is to create a “double master” or two Slide Masters in your presentation.
For example, a light background in a darkened auditorium assaults the eyes, so create a second identical Slide Master by right-clicking on the top master slide and choose Duplicate. Change the background to black or dark blue and tweak any font or object colors. Delete the one you don’t want. Slides will automatically adopt the other master.