Fix gradient, regression from #7927 (#7963)

app/javascript/styles/mastodon/forms.scss

@@ -378,7 +378,7 @@ code {
         right: 0;
         bottom: 1px;
         width: 5px;
-        background-image: linear-gradient(to right, rgba($classic-base-color, 0), $classic-base-color);
+        background-image: linear-gradient(to right, rgba($ui-base-color, 0), $ui-base-color);
       }
     }
   }

app/javascript/styles/mastodon/rtl.scss

@@ -217,7 +217,7 @@ body.rtl {
     &::after {
       right: auto;
       left: 0;
-      background-image: linear-gradient(to left, rgba($classic-base-color, 0), $classic-base-color);
+      background-image: linear-gradient(to left, rgba($ui-base-color, 0), $ui-base-color);
     }
   }