Google Fonts

add_action('admin_enqueue_scripts', function () {

    // Load Google Font Inter
    wp_enqueue_style(
        'custom-admin-inter-font',
        'https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700&display=swap',
        [],
        null
    );

    // Force font everywhere in admin
    $custom_css = '
    
    * {
        font-family: "Inter", sans-serif !important;
    }

    .editor-styles-wrapper,
    .block-editor-writing-flow,
    .block-editor-rich-text__editable,
    .edit-post-visual-editor,
    .interface-interface-skeleton__content {
        font-family: "Inter", sans-serif !important;
    }

    ';

    wp_add_inline_style('custom-admin-inter-font', $custom_css);

});

Segoe UI / Microsoft

add_action('admin_enqueue_scripts', function () {

    // Force Segoe UI font everywhere in admin
    $custom_css = '
    
    * {
        font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif !important;
    }

    .editor-styles-wrapper,
    .block-editor-writing-flow,
    .block-editor-rich-text__editable,
    .edit-post-visual-editor,
    .interface-interface-skeleton__content {
        font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif !important;
    }

    ';

    wp_register_style('custom-admin-font', false);
    wp_enqueue_style('custom-admin-font');

    wp_add_inline_style('custom-admin-font', $custom_css);

});