Add dark border between category headers in Winners table

- Match header borders (2px #999) with data row category separators

Co-authored-by: openhands <openhands@all-hands.dev>

content.py

@@ -1022,14 +1022,14 @@ h3 .header-link-icon {
     font-size: 13px;
     color: var(--color-text-dark);
     border-bottom: 2px solid var(--color-primary-accent);
-    border-right: 1px solid #eee;
+    border-right: 2px solid #999;
     white-space: nowrap;
 }
 
 .dark .winners-unified-table .category-header {
     color: #fff;
     border-bottom-color: var(--color-primary-accent);
-    border-right-color: var(--color-neutral-700);
+    border-right-color: var(--color-neutral-500);
 }
 
 .winners-unified-table .category-header:last-child {