It’s possible a theme or plugin is already loading FontAwesome, so check for that first. It’s pretty common.

I followed this tutorial:
https://www.smashingmagazine.com/2016/01/create-customize-wordpress-child-theme/#what-are-child-themes-and-why-use-them

As functions.php is likely not well configured, placing css code in style.css does nothing, so i currently got the extra css again in “Additional CSS” and the same error appears of course.

With functions.php properly configured it should be pulling the css from style.css in onetone_child theme folder right?

I am totally new to php. This is the code i have if you are able to point me to the right direction:

<?php

add_action( 'wp_enqueue_scripts', 'enqueue_parent_styles' );

function enqueue_parent_styles() {
   wp_enqueue_style( 'parent-style', get_template_directory_uri().'/style.css' );
}

<?php
function my_theme_enqueue_styles() {

    $parent_style = 'parent-style'; // This is 'twentyfifteen-style' for the Twenty Fifteen theme.

    wp_enqueue_style( $parent_style, get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array( $parent_style ),
        wp_get_theme()->get('Version')
    );
}
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );

That example, which enqueues both the parent and child theme stylesheets is from the Codex: https://codex.www.ads-software.com/Child_Themes#How_to_Create_a_Child_Theme

Thanks!