If you are unsure of the venue in which you’ll be presenting your slides, you might want to have a “Plan B” Slide Master.
For example, in large darkened auditoriums with huge screens, a darker or shaded background might be preferable. Before packing up to give your presentation, access your Slide Master (View, Master Views group, Slide Master). Right-click on the master slide, at the top, and choose Duplicate the Slide Master. Change the background on the new master and make any other needed adjustments. This way, it’s just a matter of changing the layouts of the slides to the darker background choices.