I'm not sure I can explain with words but with help of screenshot I will. How can I remove all decorations (frame and solid fill) when item is selected in QTreeView? Preferably with using style sheets.
What should show-decoration-selected do? There is no style property with this name.
Try ::item:selected and the other suggestions. If these don't work, you can't do it with style sheet and you need to change your item or your model directly