Outliner for the outliners
This seems to work for the latest version:
.tree-item-collapse {
padding: 2px 15px 2px 15px;
left:1.5px;
}
div.tree-item-inner{
position:relative;
padding-left: 5px;
}
.outline .tree-item-children {
margin-left: 20px;
border-left: 1px solid rgba(118,158,165,0.2);
border-radius: 4px;
transition:all 0.5s ease-in-out;
}
.outline .tree-item-children:hover {
border-left-color: rgba(118,158,165,0.4);
}
.tree-item-self .collapse-icon {
margin-left: -8px;
}
EDIT: the lines weren’t showing under the arrows properly, so added some extra CSS that rectified it for me. Not an expert with CSS, just building upon what others have posted and winging it! 
I just need to see if there’s a way to get the items to adopt the header or tag colours in this view now.