[Frontend] Form styling

#772
Added ".l-controls-under" styling to allow
channel-selector layout;
Added new hint color constants and refined
style defs;

platform/commonUI/general/res/sass/forms/_elems.scss

@@ -87,9 +87,7 @@
 			}
 		}
 
-		.field-hints {
-			color: darken($colorBodyFg, 20%);
-		}
+		.hint, .field-hints {  color: $colorFieldHint;  }
 
 		.selector-list {
 			// Used in create overlay to display tree view
@@ -130,6 +128,14 @@
     }
 }
 
+.l-controls-under.l-flex-row {
+    // Change to use column layout
+    @include flex-direction(column);
+    .flex-elem {
+        margin-bottom: $interiorMarginLg;
+    }
+}
+
 .no-validate {
     .form .form-row >.label {
         padding-right: 0;