Hi everyone,
It'll depend on your style and what you've got configured: UI.X itself has a background color change by default, but it looks like the style you're using has a box shadow on hover.
Either way, the easiest is going to be editing the style properties for Node Body (Hover) under Appearance -> Style Properties -> Node/forum list.
In your case, I'd add box-shadow: none in the extra less/css field for that.
If it's the background color changing, just remove the background color from the background property.