The theme editor allows you to load custom fonts from Typekit, Google Fonts or your own host. These options are all available through the theme CSS editor.

Typekit and Google Fonts
To load a font from Typekit or Google, open the theme CSS editor and follow the instructions in this article: http://help.slides.com/knowledgebase/articles/1077976

Importing from another host
If you want to import a font from another host you can do this using the CSS editor. For example, the following CSS loads the "Comofrtaa" font from Google Fonts and applies it to all h1, h2 and h3 elements.

@import url(//fonts.googleapis.com/css?family=Comfortaa:400,300,700);

.slides h1, .slides h2, .slides h3 {
    font-family: 'Comfortaa', sans-serif;
}