<stylesheet>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Classic Three-Column Footer</title>
    <style>
        /* 
         * Using CSS variables for easy theming and maintenance.
         * Mobile-first responsive design.
         */
        :root {
            --footer-bg: #222529;
            --footer-text-color: #adb5bd;
            --footer-heading-color: #ffffff;
            --footer-link-color: #adb5bd;
            --footer-link-hover-color: #ffffff;
            --footer-border-color: #3b3f44;
        }

        /* Basic body styles for demonstration */
        body {
            font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
            margin: 0;
            display: grid;