1、方法一：

#include <QStyleFactory>

ui.treeView->setStyle(QStyleFactory::create("windows"));

2、方法二：

 QString strtyle2 = R"(
QTreeView::branch:has-siblings:!adjoins-item {
    border-image: url(:/TreeViewDemo/res/vline.png);
}
QTreeView::branch:has-siblings:adjoins-item {
    border-image: url(:/TreeViewDemo/res/branch-more.png);
}

QTreeView::branch:!has-children:!has-siblings:adjoins-item {
    border-image: url(:/TreeViewDemo/res/branch-end.png);
}
QTreeView::branch:has-children:!has-siblings:closed,
QTreeView::branch:closed:has-children:has-siblings
 {
border-image: none;
image: url(:/TreeViewDemo/res/branch-closed.png);
}

QTreeView::branch:open:has-children:!has-siblings,
QTreeView::branch:open:has-children:has-siblings
 {
border-image: none;
image: url(:/TreeViewDemo/res/branch-open.png);
}
 QTreeView::branch:open:has-children:!has-siblings:selected,
QTreeView::branch:open:has-children:has-siblings:selected {
border-image: none;
border-left:5px solid blue;
}
)";
    ui.treeView->setStyleSheet(strtyle2);

样式表可参考：

Qt Style Sheets Examples | Qt Widgets 5.15.16

效果如下所示：