Example of overriding Style Customizer fonts with a Typekit font. (For additional context see http://www.carriedils.com/typekit-rainmaker/)

example-typekit.css

/* 
Note the .site-container class - this adds extra specification to areas
where the Style Customizer is applying styles. You can directly target 
other styles that come directly from the theme (hint: you can identify them 
because they don't include the .gppro-custom class)
------*/
body.gppro-custom .site-container,
body.gppro-custom .site-container .genesis-grid .entry-meta,
body.gppro-custom .site-container .entry-header .entry-meta,
body.gppro-custom .site-container .site-footer p,
body .search-form > input,
body .search-form > textarea,
body .ninja-forms-form-wrap input,
body .ninja-forms-form-wrap textarea,
input.input,
input.input-text,
#comment {
    font-family: 'Proxima Nova', sans-serif;
}