Make the file tree more visible as you scroll sections

atlassian-confluence-file-tree.css

.plugin_pagetree .plugin_pagetree_children_list>li .plugin_pagetree_children_list {
    padding-left: 16px;
    background-image: linear-gradient(to right, #d0d0e233, #ffffff12);
    border-top-left-radius: 12px 8px;
    border-bottom-left-radius: 12px 8px;
    border-width: 1px;
    border-color: #b4d3ef;
    border-left-style: solid;
}
.plugin_pagetree_childtoggle_container>.no-children.icon {
    background: #ff6300 !important;
}

.expand-control-icon, .ui-tree li.closed>.click-zone, #children-section.children-hidden a.children-show-hide.icon, .icon-section-closed {
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' height='100%25' width='100%25' version='1.1' id='svg3025' viewBox='0 -256 1792 1792'%3E%3Cg transform='matrix(1,0,0,-1,349.28814,1330.9831)' id='g3027'%3E%3Cpath d='m 1099,704 q 0,-52 -37,-91 L 410,-38 q -37,-37 -90,-37 -53,0 -90,37 l -76,75 q -37,39 -37,91 0,53 37,90 l 486,486 -486,485 q -37,39 -37,91 0,53 37,90 l 76,75 q 36,38 90,38 54,0 90,-38 l 652,-651 q 37,-37 37,-90 z' id='path3029' style='fill:#ff6300'/%3E%3C/g%3E%3C/svg%3E") !important;
}

.expand-control-icon, .ui-tree li.closed>.click-zone, #children-section.children-hidden a.children-show-hide.icon, .icon-section-opened {
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' height='100%25' width='100%25' version='1.1' id='svg3025' viewBox='0 -256 1792 1792'%3E%3Cg transform='matrix(1,0,0,-1,349.28814,1330.9831)' id='g3027'%3E%3Cpath d='m 1099,704 q 0,-52 -37,-91 L 410,-38 q -37,-37 -90,-37 -53,0 -90,37 l -76,75 q -37,39 -37,91 0,53 37,90 l 486,486 -486,485 q -37,39 -37,91 0,53 37,90 l 76,75 q 36,38 90,38 54,0 90,-38 l 652,-651 q 37,-37 37,-90 z' id='path3029' style='fill:#ff6300'/%3E%3C/g%3E%3C/svg%3E") !important;
    transform: rotate(90deg);
}

Note: an example of how it displays