You can do either with changes in extra.less, e.g.
Less:
.structItemContainer-group.structItemContainer-group--sticky
{
border-bottom: 2px solid red;
}
Thick red border between the sticky group and the non sticky group.
Less:
.structItemContainer-group.structItemContainer-group--sticky
{
.structItem--thread
{
background-color: lightblue;
}
}
Different background colour for sticky threads.