I am able to set style sheets when I do this (normal styling):

QTabWidget::pane {
 <style here>
}

or when I do this (using an id)

#id QTabWidget::tab-bar{
 <style here>
}

But nothing happens when I do this (using an id and ::pane)

#id QTabWidget::pane{
 <style here>
}

It appears you can't do "#id QTabWidget::pane", but this is allowed for all other css tags I've tried. I am currently working around this by leaving the id off, but this has the undesired effect of cascading down.