Why can't I custom q-tree__node--selected?

[weide]

I want to custom selected node’s style,so I write some css like this:

    .q-tree {

      .q-tree__node {
        .q-tree__node--selected {
          background: blue;
        }

        .q-tree__node--selected {
          .q-tree__node-header-content {
            .node-label {
              color: blue !important;
            }
          }
        }

      }
    }

It seems ‘note-label’ works fine,but q-tree__node–selected not worked…