Hi Brian -

There are a variety of ways to do this, from making your own to downloading ones from other sites. Many people/sites use themes and templates interchangeably, so searching for powerpoint templates will get you more results.

If you want to create your own theme from scratch, you can do that by creating a background, layout, and color scheme on the master slide and then under themes, you can save the theme and name it whatever you want.

Another trick is that if you are using a theme/template you like or one that you reuse constantly for work, you can save that theme also and it will show up as an available design option on your menu ribbon with the default themes.

Based upon your question I'm not sure which route you want to go, so hopefully this helps.