.ztree .level0 {
color: blue;
}
.ztree .level1 {
color: red;
}
.ztree .level2 {
color: green;
}

fontCss说明里说的很明白了。。
var setting = {
view: {
fontCss: setFontCss
}
};
function setFontCss(treeId, treeNode) {
var color;
switch (treeNode.level) {
case 0:
color = { color: "red" };
break;
case 1:
color = { color: "blue" };
break;
case 2:
color = { color: "green" };
break;
default:
color = {};
}
return color;
};