[jQuery] jQuery TreeView : impossible to set a property...

Hi again,
In a jQuery TreeView, I'd like to change background-color for some
elements...
So, I've written this code :
function refreshStriping () {
    var collect_ul, fils, petits_fils, i, j, k;
    // On récupère la collection de tous les 'ul'
    collect_ul = document.getElementsByTagName("ul");
    for (i=0;i<collect_ul.length;i++) {
        fils = collect_ul[i].childNodes;
        for (j=0;j<fils.length;j++) {
            if (fils[j].tagName == "li") {
                petits_fils = fils[j].childNodes;
                for (k=0;k<petits_fils.length;k++) {
                    if (petits_fils[k].tagName = "span") {
                        petits_fils[k].style = "background-color:yellow";
                    }
                }
            }
        }
    }
}
But I have an error :
- setting a property that has only a getter !
I don't understand why I can't...
Do you have any clue ?
Thanks a lot.
David.