ul.tree li {
    list-style-type: none;
    position: relative;
    padding : 6px;
}

ul.tree li ul {
    display: none;
}

ul.tree li.open > ul {
    display: block;
}

ul.tree li a {
    color: black;
    text-decoration: none;
}

ul.tree li a:before {
    height: 1em;
    padding:0 .1em;
    font-size: .8em;
    display: block;
    position: absolute;
    left: -1.3em;
    top: .2em;
}

ul.tree li > a:not(:last-child):before {
    content: '+';
    font-weight: bold;
    color: green;
}

ul.tree li.open > a:not(:last-child):before {
    content: '-';
    color: red;
    font-weight: bold;
}