fembooru/ext/holiday/theme.php

<?php
class HolidayTheme extends Themelet
{
    public function display_holiday($date)
    {
        global $page;
        if ($date) {
            $csssheet = "<link rel='stylesheet' href='".get_base_href()."/contrib/holiday/stylesheets/";

            // April Fools
            // Flips the entire page upside down!
            // TODO: Make it possible for the user to turn this off!
            if (date('d/m') == '01/04') {
                $csssheet .= "aprilfools.css";
            }

            $csssheet .= "' type='text/css'>";
            $page->add_html_header("$csssheet");
        }
    }
}