This page explains how to customize the selection marker used to highlight the current selection in an inventory display.

Selection Marker

The Selection Marker is an object that will constantly move to position itself on top of your currently selected slot. To create one, all you need is to create a new Image object inside your Canvas, and add an InventorySelectionMarker to it. From its inspector you’ll be able to define the transition speed and minimal transition distance (which is best left at its default value). You can also add an animator to it if you want.