css: fix login form for classic theme

The footer had an ugly dark overlay

src/shared/styles/themes/classic.scss

@@ -24,7 +24,7 @@
     // --------------
     --foreground-color: #666;
     --background-color: #efefef;
-    --background-color-rgb: 102, 102, 102;
+    --background-color-rgb: 239, 239, 239;
 
     --chat-rgb: var(--green-rgb);
     --muc-rgb: var(--orange-rgb);