TreeItem - bold only the parent item in a Tree?

Hello,  in treeview I would like to bold only the parent item, and keep sub-items in normal size.

I tried to use css in this way, but it does not work:
.gwt-Tree{
    font-weight: bold;
}

.gwt-TreeItem {
    font-weight: normal;
}

.gwt-Tree .gwt-TreeItem-selected {
    font-weight: normal;
}

Is there a way to get there?

Thank you in advance

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