Alex,
It is a CSS issue as the toc is expecting a standard dojo them like claro and you are using bootstrap.
Just add this to your style section:
.dijitTreeExpando {
background-image: url("http://js.arcgis.com/3.14/dijit/themes/claro/images/treeExpandImages.png");
width: 16px;
height: 16px;
background-position: -35px 0;
}