The default is for the selected row to be colored gray if the QTableWidget does not have focus, and orange if it does have focus. I would like to, instead, make the selected
You almost had it. Technically speaking, you're adjusting the selection color of the items within your table widget, so:
QTableWidget::item{ selection-background-color: red}
should do the trick.
Alternatively:
QTableWidget::item{ background-color: blue }
QTableWidget::item:selected{ background-color: red }