update placeholder text to meet contrast requirement

bigbluebutton-html5/client/main.html

@@ -74,6 +74,12 @@ with BigBlueButton; if not, see <http://www.gnu.org/licenses/>.
     [hidden]:not([hidden="false"]) {
       display: none !important;
     }
+
+    textarea::-webkit-input-placeholder,
+    input::-webkit-input-placeholder {
+      color: var(--palette-placeholder-text);
+      opacity: 1;
+    }
   </style>
   <script>
     document.addEventListener('gesturestart', function (e) {

bigbluebutton-html5/imports/ui/stylesheets/variables/palette.scss

@@ -1,4 +1,6 @@
 :root {
+  --palette-placeholder-text: #7a7a7a;
+
   --color-white: #FFF;
   --color-off-white: #F3F6F9;