Pull web fonts into Shopify's checkout from the theme settings, through a sass file located outside the theme directory. Why? ... because its cool.

checkout.scss

/*
 * Checkout.scss sits outside the Shopify theme folder so that we can pull in Boostrap and our checkout reset via scss imports.
 * This will be compiled into checkout.css.liquid and saved in our theme assets folder.
 * As this will be compiled into liquid we need to escape our liquid tags to prevent our compiler having a meltdown.
 *
 * You need to do this across two files otherwise you'll end up with an empty import rule at the top of your checkout.css.liquid 
 */
 
 // import the compiled font_import.css.liquid from our assets folder.
 // note the escaped liquid.
 @import url(#{'{{ "font_import.css" | asset_url }}'});
 
 // output: @import url(//cdn.shopify.com/s/files/1/1234/5678/t/5/assets/font_import.css);

font_import.css.liquid

{% comment %}
/*
 * font_import.css.liquid lives inside theme/assets/ and will be compiled into css by Shopify.
 *
 * IMPORTANT NOTE: This will only work if we are using a _non_ javascript way of importing a web font.
 * e.g. <link type="text/css" rel="stylesheet" href="http://fast.fonts.net/cssapi/1234-5678.css"/>
 *
 * 1. Get our custom-font-embed-code from the theme settings.
 * 2. Check if its a link
 * 3. Split it up on the spaces and loop through the parts
 * 4. Find the href
 * 5. Remove the pieces of the string we don't need
 * 6. pass that to the css import
 */
{% endcomment %}

{% if settings.custom-font-embed-code != blank %}
  {% if settings.custom-font-embed-code contains '<link' %}
    {% assign link = settings.custom-font-embed-code | split: ' ' %}
    {% for attr in link %}
      {% if attr contains 'href' %}
        {% assign import = attr | remove: 'href="' | remove: '"/>' | remove: 'https:' | remove: 'http:' %}
          @import url({{ import }});
      {% endif %}
    {% endfor %}
  {% endif %}
{% endif %}

{% comment %}
/*
 * output: @import url(//fast.fonts.net/cssapi/1234-5678.css);
 */
{% endcomment %}