The problem is at your main.css, you have
#leftCol img {
width: 100%;
}
Which overwrites
.dijitFolderOpened, .dijitIconFolderOpen, .dijitIconError {
background-image: url("../../icons/images/commonIconsObjActEnabled.png");
width: 16px;
height: 16px;
}
You need resolve this in main.css
by either removing your style, or changing it to a more specific rule; i.e. instead of #leftCol img
, use #leftCol .yourClass
.