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Hi, thanks for answering! Unfortunately it didn't work, maybe there was something else wrong with the code snippet. I got another one that let me inline css with a shortcode. Here's the workaround in case anyone needs such a thing :

function get_latest_project() {
    $current_post_id = get_the_ID();

    $args = array(
        'post_type'      => 'projects',
        'posts_per_page' => 1,
        'post_status'    => 'publish',
        'order'          => 'DESC',
        'orderby'        => 'date',
    );

    $query = new WP_Query( $args );
    
    $latest_post_id = null;
    if ( $query->have_posts() ) {
        $query->the_post();
        $latest_post_id = get_the_ID(); 
        wp_reset_postdata();
    } 
    
    $css = '<style>/* not latest */</style>';
    if ($latest_post_id AND $current_post_id == $latest_post_id) {
        $css = '<style>/* latest */</style>';
    }

    return $css;
}
add_shortcode( 'latest_project', 'get_latest_project' );