got the same problem, using qt4.4.6.2 at ubuntu 10.04 linux 2.6.32-22.37

Qt Code: 
  1. QToolBar* mainToolBar;
  2.  
  3. ...
  4.  
  5.     mainToolBar = new QToolBar(this);
  6.     mainToolBar->setObjectName(QString::fromUtf8("mainToolBar"));
  7.     mainToolBar->setStyleSheet("QToolBar { background:red; }");
  8.     mainToolBar->setMovable(false);
  9.     mainToolBar->setAllowedAreas(Qt::TopToolBarArea);
  10.     mainToolBar->setIconSize(QSize(56, 56));
  11.     mainToolBar->setToolButtonStyle(Qt::ToolButtonTextUnderIcon);
  12.     mainToolBar->setFont(font);
  13. ...
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but the bar is still light-grey (default)