fix: Dark mode footer styling issues

Fixes two dark mode display issues:
- Add html background-color in dark mode to prevent white background showing at bottom of page
- Remove hardcoded black color from footer GitHub link text to respect theme colors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: Ian Clarke

hugo-site/themes/freenet/assets/css/freenet.css

@@ -224,6 +224,10 @@ p.page-end-spacer {
 
 /* Dark mode styles */
 @media (prefers-color-scheme: dark) {
+    html {
+        background-color: #121212;
+    }
+
     body {
         background-color: #121212;
         color: #e0e0e0;

hugo-site/themes/freenet/layouts/_default/baseof.html

@@ -21,7 +21,7 @@
                 <a href="https://github.com/freenet/web"><img
                         src="https://github.githubassets.com/images/modules/logos_page/GitHub-Mark.png" alt="GitHub"
                         style="width: 20px; height: 20px; vertical-align: middle;"> <span
-                        style="color: black; font-weight: 600;">freenet/web</span></a>.
+                        style="font-weight: 600;">freenet/web</span></a>.
     </footer>
 </div>
 </body>