fix(list): subtitle contrast color in light and dark theme

Author: MPopov

projects/igniteui-angular/src/lib/core/styles/themes/schemas/light/_list.scss

@@ -247,15 +247,15 @@ $_indigo-list: extend(
         ),
 
         item-subtitle-color: (
-            igx-color: ('grays', 700)
+            igx-color: ('grays', 800)
         ),
 
         item-subtitle-color-hover: (
-            igx-color: ('grays', 700)
+            igx-color: ('grays', 800)
         ),
 
         item-subtitle-color-active: (
-            igx-color: ('grays', 700)
+            igx-color: ('grays', 800)
         ),
 
         item-thumbnail-color: (