Remove Ubuntu font from WP4.6 system font stack

wpgetridofubuntusystemfont.php

<?php

/**
 * Plugin name: Get rid of Ubuntu font
 * Description: Attempt to override the 4.6 system font stack to get rid of Ubuntu font.
 * Author: Otto Rask
 * Author URI: https://github.com/rask
 * License: MIT
 */

/**
 * NOTICE
 *
 * This was created from a personal dislike against the Ubuntu system font, which is
 * now used by default on Ubuntu systems when WordPress 4.6 is installed. If you
 * wish to use some other system sans-serif font for WP core styles, use this.
 *
 * This is supposed to be used as a mu-plugin, so it loads on all requests. Copy the
 * contents of this file to a PHP file inside the mu-plugins directory.
 */

if (!defined('ABSPATH')) {
    exit;
}

/**
 * Get the font stack that should replace the WP core font stack. Otherwise the same
 * as in 4.6, but with `Ubuntu` removed.
 *
 * @return string
 */
function grou_get_new_font_stack()
{
    return '-apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Cantarell, "Helvetica Neue", sans-serif';
}

/**
 * Style tag for frontend.
 *
 * Used to reset admin bar font stack.
 *
 * @return void
 */
function grou_custom_css()
{
    $new = grou_get_new_font_stack();

    echo <<<EOF
<style type="text/css">
    #wpadminbar, #wpadminbar * {
        font-family: {$new};
    }
</style>
EOF;
}

/**
 * Style tag for admin panel.
 *
 * Reset font stacks globally in admin.
 *
 * @return void
 */
function grou_custom_admin_css()
{
    $new = grou_get_new_font_stack();

    echo <<<EOF
<style type="text/css">
    body, #wpadminbar, #wpadminbar *,
    body .media-modal, body .media-frame,
    .media-modal-content .media-frame input[type="text"], .media-modal-content .media-frame input[type="password"],
    .media-modal-content .media-frame input[type="number"], .media-modal-content .media-frame input[type="search"],
    .media-modal-content .media-frame input[type="email"], .media-modal-content .media-frame input[type="url"],
    .media-modal-content .media-frame textarea, .media-modal-content .media-frame select {
        font-family: {$new};
    }
</style>
EOF;
}

/**
 * Style tag for login screen.
 *
 * Reset font stacks in login.
 *
 * @return void
 */
function grou_custom_login_css()
{
    $new = grou_get_new_font_stack();

    echo <<<EOF
<style type="text/css">
    body {
        font-family: {$new};
    }
</style>
EOF;
}

// Hook in frontend. Use footer scripts as admin bar styles are loaded preeetty late.
add_action('wp_print_footer_scripts', 'grou_custom_css', 999);

// Hook in admin. Late priority to ensure overriding.
add_action('admin_print_styles', 'grou_custom_admin_css', 999);

// Hook in login.
add_action('login_footer', 'grou_custom_login_css', 999);

Some spots where this needs to step in too:

• Media library modal
• Login page

Will add in when I have the time.

Added font reset for the login screen.

Added font reset to media modal and frames. Might have missed some spots.