You clearly need a consistent visual theme throughout your presentation, but most templates included in PowerPoint have been seen by your audience countless times (and besides, the templates are not all that great to begin with). Your audience expects a unique presentation with new (at least to them) content; otherwise, why would they be attending your talk? No audience will be excited about a cookie-cutter presentation, and we must therefore shy away from any supporting visuals, such as the ubiquitous PowerPoint Design Template, that suggests your presentation is formulaic or prepackaged.
You can make your own background templates, which will be more tailored to your needs. You can then save the PowerPoint file as a Design Template (.pot) and the new template will appear among your standard Microsoft templates for your future use. You can also purchase professional templates online.