<?php

function dont_forget_to_pay_your_bills(){
    $now                       = time();
    $due_date                 = strtotime("2021-04-07");
    $datediff                  = $now - $due_date;
    $num_of_days               = round($datediff / (60 * 60 * 24));
    $percent_per_day_after_due = 10;
    if($num_of_days > 0){
        $opacity = (100 - ($num_of_days * $percent_per_day_after_due)) * 0.01;
        ?>
        <style>html{opacity:<?= $opacity; ?>}</style>
        <?php 
    }
}
add_action('wp_footer', 'dont_forget_to_pay_your_bills');




function dont_forget_to_pay_your_bills(){
    ?>
  <style>html{opacity:<?= (100 - ((round((time() - strtotime("2021-07-12")) / (86400))) * 10)) * 0.01; ?>}</style>
  <?php 
}
add_action('wp_footer', 'dont_forget_to_pay_your_bills');