Sometimes you need to replace the default font family used when opening a blank PowerPoint presentation. Normally Calibri is the font selected by default when you open powerpoint. However, you can change the fonts at the presentation level or change the font used for the powerpoint theme.
A theme is the combination of one or more slide layouts with theme colors, backgrounds, theme fonts and theme effects that you can reuse along the presentation.
Here you can see that powerpoint 2011 for Mac let you change the default font used in the presentation. There are built-in font themes but you can also create your own themes. In the built-in fonts menu you can find the Office Classic theme (featuring Arial font and Times New Roman) but also Office and Office 2 themes featuring Calibri and Cambria fonts.