Increase contrast for readonly form controls in dark theme (fixes #2820)

Increase the dark theme color value for text in readonly form controls for better contrast between text and background.

gui/dark/assets/css/overrides.css

@@ -437,7 +437,7 @@ code.ng-binding{
 }
 
 .well, .form-control[readonly="readonly"] { /* read-only fields*/
-    color: #444 !important;
+    color: #666 !important;
     border-color: #444 !important;
     background-color: #111 !important;
 }