Polish footer: blend Ko-fi button into page background

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: asimone

index.html

@@ -308,10 +308,8 @@
   font-family: 'Lato', sans-serif;
   font-size: .78rem;
   font-weight: 700;
-  color: #fff;
-  background: #29abe0;
-  padding: 4px 10px;
-  border-radius: 5px;
+  color: var(--gold);
+  background: var(--bg);
   text-decoration: none;
   white-space: nowrap;
   transition: opacity .15s;