Fix #2253, improper disabled form field rules

src/definitions/collections/form.less

@@ -516,18 +516,18 @@
        Disabled
 ---------------------*/
 
-.ui.form .field :disabled,
-.ui.form .field.disabled {
-  opacity: 0.5;
+.ui.form .disabled.fields .field,
+.ui.form .disabled.field,
+.ui.form .field :disabled {
+  opacity: @disabledOpacity;
 }
 .ui.form .field.disabled label {
-  opacity: 0.5;
+  opacity: @disabledLabelOpacity;
 }
 .ui.form .field.disabled :disabled {
   opacity: 1;
 }
 
-
 /*--------------
     Loading
 ---------------*/

src/themes/default/collections/form.variables

@@ -98,6 +98,9 @@
 @textAreaFocusBoxShadow: @inputFocusBoxShadow;
 @textAreaFocusBorderRadius: @inputFocusBorderRadius;
 
+/* Disabled */
+@disabledLabelOpacity: @disabledOpacity;
+
 /* Error */
 @formErrorColor: @negativeTextColor;
 @formErrorBorder: @negativeBorderColor;