Merge pull request #1036 from jitsi/fix-placeholder-style

Fixes placeholder color

css/_base.scss

 }
 
 @include placeholder {
-    color: $inputColor;
+    color: $placeHolderColor;
 }
 
 textarea {

css/themes/_light.scss

 $inputBackground: #fff;
 $inputBorderColor: #ccc;
 $inputColor: #333;
+$placeHolderColor: #a7a7a7;
 $defaultDarkSelectionColor: #ccc;
 $buttonFontWeight: 400;
 $labelFontWeight: 400;