To customize the appearance of the Selector or Proximity Selector's UI:
1. The Dialogue Manager GameObject has an Instantiate Prefabs component. Remove the "Basic Standard UI Selector Elements":
2. Add "Basic Standard UI Selector Elements" to the Dialogue Manager's Canvas. Activate the Main Selector Panel:
Then customize the appearance like any other Unity UI. When you're done, you can deactivate the Main Selector Panel if you want, to keep the screen uncluttered while you're working.
You can also optionally save it as a new prefab, remove it from the Canvas, and assign it back to the Instantiate Prefabs component if you prefer.