At this point it's useful do decide on a colour scheme and background for your presentation.

Go to FORMAT on your menu bar.

Spend a little time experimenting with different Slide Layouts, Colour Schemes and Backgrounds until you find a combination of these enhancements which suits your presentation

Try out the different options for creating effects with gradient fills, patterns, textures, and combinations of fonts, fills, texts and lines, shadows and accents.

Again, when in doubt, adopt a conservative approach.  ALWAYS keep in mind your audience, the time of the presentation and the venue when deciding on a background and colour scheme.

As Roseann will have pointed out to you in Rhetoric and Speech, backgrounds that appear to you to look really striking as you design your presentation, may in fact compromise the effectiveness of your message.