Return customizer setting value in functions.php

The solution was to get the customizer value inside the function, not passing the value to the function.

    function hn_add_inline_css() {
        $breakpoint = get_theme_mod( 'setting_navigation-breakpoint', '768' );

        $custom_css="
            @media (min-width: " . $breakpoint . 'px) {
              .form-inline .form-group {
                display: inline-block;
                margin-bottom: 0;
                vertical-align: middle;
              }
              ...