When QTableView edit control is visible for the current item the shylesheet of the edit takes place. When there is no active edit control in the QTableVie
Qt style sheets support sub-controls and pseudo states, you can use it to improve your customization. (see http://qt-project.org/doc/qt-5/stylesheet-reference.html#list-of-pseudo-states )
In this case you can use the ::item sub-control and the :focus pseudo state (the "current" pseudo state doesn't exist, but the :focus does the same).
This is an example that you can use:
QTableView::item:focus
{
selection-background-color: yellow;
}

See also http://qt-project.org/doc/qt-5/stylesheet-examples.html#customizing-qtreeview