Fix Directory Browser accessibility (#63393)

Applied contrast recommendations from accessibility insights.

Author: sebastienros

src/Middleware/StaticFiles/src/HtmlDirectoryFormatter.cs

@@ -79,7 +79,7 @@ header h1 {
         vertical-align: bottom;
         padding: 10px 5px 5px 5px;
         font-weight: 400;
-        color: #a0a0a0;
+        color: #737373;
         text-align: center; }
     #index td { padding: 3px 10px; }
     #index th, #index td {
@@ -91,7 +91,7 @@ header h1 {
     #index th:last-child, #index td:last-child {
         border-right: 1px transparent solid; }
     #index td.length, td.modified { text-align:right; }
-    a { color:#1ba1e2;text-decoration:none; }
+    a { color:#127aac;text-decoration:none; }
     a:hover { color:#13709e;text-decoration:underline; }
   </style>
 </head>