You’re looking for a:hover – that’s the style for a link when someone is hovering over it. So I think if you add the following it will all work as you’d like and be the same colour as your other links. This changes the default link hover colour. There might be something somewhere tucked away that overwrites it in certain circumstances but this should work on the forum index page at least.
a:hover{
color: #31485a;
}