CSS: Sticky table headers (fix #918)

designs/pappu687/adminer.css

@@ -140,6 +140,10 @@ tbody tr:hover td,tbody tr:hover th{
     background:#edf4ff
 }
 
+thead {
+	top: 50px;
+}
+
 thead th, thead td {
     text-align:center;
     vertical-align:middle;