By default, when you mouse over a conversation option it will highlight the selection green. I can't figure out where this option is, but I'm trying to disable it.
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Open the Menu Manager and select the Conversation menu. The next part depends on the Menu's "Source" property, which controls how it is rendered.
If set to Adventure Creator, select DialogueList from its list of elements below. The green texture can then be found assigned in its Highlight texture property.
If set to Unity Ui Prefab, select the Linked Canvas prefab and double-click the prefab in the Project window to open it. Navigate through its Hierarchy to find the "btnOption" objects - which each represent a different option. For each, view its Button component in the Inspector, and set the Highlighted Color in its Transition settings.