By default it uses the background/border-color(from the bottom) of SecondaryContent(Style Properties -> Building Blocks).
You can also edit it specifically by going to: Style properties -> Forum / Node List -> Last Post.
For the icon, you'll need to edit the xenforo-ui-sprite.png, or you can change it so 0 opacity via style properties -> forum / node list -> tiny icon
Can also put display: none if you want to remove it and not show on hover.