HelloWorld.pro

#-------------------------------------------------
#
# Project created by QtCreator 2014-08-26T10:07:27
#
#-------------------------------------------------

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = HelloWorld
TEMPLATE = app


SOURCES += main.cpp\
        mainwindow.cpp

HEADERS  += mainwindow.h

FORMS    += mainwindow.ui

main.cpp

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QObject::connect(ui->pushButton, SIGNAL(clicked()),
                     this, SLOT(changeText()));
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::changeText()
{
    ui->label->setText("Hello World!");
    ui->label->setProperty("clicked", true);

    updateStylesheet(ui->label);
}

void MainWindow::updateStylesheet(QWidget *widget)
{
    widget->style()->unpolish(widget);
    widget->style()->polish(widget);
    widget->update();
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

public slots:
    void changeText();

private:
    void updateStylesheet(QWidget *widget);

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <property name="styleSheet">
   <string notr="true">QPushButton {
  color: blue;
}

QLabel {
  color: green;
}

QLabel[clicked=&quot;true&quot;] {
  color: red;  
}

s</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <layout class="QHBoxLayout" name="horizontalLayout">
    <item>
     <layout class="QVBoxLayout" name="verticalLayout">
      <item>
       <widget class="QLabel" name="label">
        <property name="font">
         <font>
          <family>Geeza Pro</family>
         </font>
        </property>
        <property name="styleSheet">
         <string notr="true"/>
        </property>
        <property name="text">
         <string>please click the button</string>
        </property>
       </widget>
      </item>
      <item>
       <widget class="QPushButton" name="pushButton">
        <property name="styleSheet">
         <string notr="true"/>
        </property>
        <property name="text">
         <string>Click me</string>
        </property>
       </widget>
      </item>
      <item>
       <widget class="QPushButton" name="pushButton_2">
        <property name="autoFillBackground">
         <bool>true</bool>
        </property>
        <property name="styleSheet">
         <string notr="true"/>
        </property>
        <property name="text">
         <string>No no click me</string>
        </property>
       </widget>
      </item>
     </layout>
    </item>
    <item>
     <widget class="QListWidget" name="listWidget">
      <item>
       <property name="text">
        <string>red</string>
       </property>
      </item>
      <item>
       <property name="text">
        <string>blue</string>
       </property>
      </item>
      <item>
       <property name="text">
        <string>green</string>
       </property>
      </item>
      <item>
       <property name="text">
        <string>yellow</string>
       </property>
      </item>
     </widget>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="menuBar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>400</width>
     <height>22</height>
    </rect>
   </property>
  </widget>
  <widget class="QToolBar" name="mainToolBar">
   <attribute name="toolBarArea">
    <enum>TopToolBarArea</enum>
   </attribute>
   <attribute name="toolBarBreak">
    <bool>false</bool>
   </attribute>
  </widget>
  <widget class="QStatusBar" name="statusBar"/>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>