If you want to add fonts to your Skin, you should add them via the fonts() method.

Simply return an array of your new font(s) in Thesis font array format, and Thesis will add your font(s) to font <select> elements in the Design Options. In addition, your font(s) will be used in typographical calculations made by the $typography object.

For example, the following code will add the Adelle font from Typekit to your Skin:

protected function fonts() {
	return array(
		'adelle' => array(
			'name' => 'Adelle',
			'family' => 'Adelle, serif'));

The name is how the font will appear in font <select> elements, and the family is what should be output in the CSS to refer to this font.

Please note that it is still up to you, the Skin developer, to include font-family references in your CSS.