DEV: Refactor `.coldmap` styles

Component is only used in mobile category lists, no need to have it available globally

Uses opacity instead of color manipulation to deliver the same effect

app/assets/stylesheets/common/base/discourse.scss

@@ -359,23 +359,6 @@ table {
   margin-left: 12px;
 }
 
-// the default for table cells in topic list
-// is $primary-medium
-// numbers get dimmer as they get colder
-.coldmap {
-  &-high {
-    color: dark-light-choose($primary-low-mid, $secondary-high) !important;
-  }
-
-  &-med {
-    color: dark-light-choose($primary-medium, $secondary-high) !important;
-  }
-
-  &-low {
-    color: dark-light-choose($primary-medium, $secondary-medium) !important;
-  }
-}
-
 .top-space {
   margin-top: 10px;
 }

app/assets/stylesheets/mobile/topic-list.scss

@@ -263,6 +263,21 @@
   .age {
     margin-left: 5px;
   }
+
+  // numbers get dimmer as they get colder
+  .coldmap {
+    &-high {
+      opacity: 0.4;
+    }
+
+    &-med {
+      opacity: 0.6;
+    }
+
+    &-low {
+      opacity: 0.8;
+    }
+  }
 }
 
 .subcategory-list-item.category {