in prosilver/theme/links.css

a.forumtitle:hover {

	text-decoration: underline;

}

a.lastsubject:hover {

	text-decoration: underline;

}

is being blocked by

.row-item a:hover {

	text-decoration: none

}

and text-decoration: none is missing the delimiter along with some in other css files
see https://www.phpbb.com/community/viewtopic.php?f=686&t=2598556 for more info