Labels-filter search input has low-contrast text and misaligned icon (#48578)

- [x] QA'd all new/changed functionality manually

For the following bug:
- https://github.com/fleetdm/fleet/issues/48564



<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Style**
* Improved the alignment of an icon within a form field for more
consistent visual layout.
* Updated input text color to a darker tone for better readability and
contrast.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

frontend/pages/hosts/ManageHostsPage/components/CustomLabelGroupHeading/_styles.scss

@@ -19,7 +19,11 @@
     .icon {
       position: absolute;
       left: 12px;
-      top: 12px;
+      top: 0;
+      height: 36px;
+      width: 16px;
+      display: flex;
+      align-items: center;
     }
   }
 
@@ -31,7 +35,7 @@
     border-radius: $border-radius;
     font-size: $small;
     padding: 9.5px 12px 9.5px 36px;
-    color: $core-fleet-blue;
+    color: $core-fleet-black;
     font-family: "Inter", sans-serif;
     font-size: $x-small;
     box-sizing: border-box;