Join David Rivers for an in-depth discussion in this video, Creating your own themes, part of PowerPoint for Mac 2011 Essential Training.
You don't have to edit XML to apply a theme. The uber-geeky folks would do that.
Sane, normal people don't. Here's a brief explanation on how to apply a theme: There's a lot more about applying and making themes in the book and in Help in PowerPoint.
You can apply themes to slide masters and slide designs, which you can save as either templates or themes. Opening your themes in the gallery A template includes more than a theme does. A template (.potx) or (.potm) includes everything within a presentation and you can use it as starting point for new presentations. A theme (.thmx) file includes preset colors, slide masters and slide designs. You can use a theme as a starting point, or apply it to an existing slide show. I am an unpaid volunteer and do not work for Microsoft. 'Independent Advisors' work for contractors hired by Microsoft.
'Microsoft Agents' work for Microsoft Support.