First of all, let's remove the style sheet from the push button by selecting it and clicking the small arrow button besides the styleSheet property. This button will revert the property to the default value, which in this case is the empty style sheet.

Then, add a few more widgets to the UI by dragging them one by one from the widget box to the form editor. I've added a line edit, combo box, horizontal slider, radio button, and a check box.

For the sake of simplicity, delete the menu bar, main toolbar, and the status bar from your UI by selecting them from the object inspector, right click, and choose Remove. Now your UI should look similar to this:

Select the main window either from the form editor or the object inspector, then right click and choose Change Stylesheet to open up the Edit Style Sheet.

Insert the following style sheet:

border: 2px solid gray;
border-radius: 10px;
padding: 0 8px;
background: yellow;

Now what you will see is a completely bizarre-looking UI with everything covered in yellow with a thick border. This is because the preceding style sheet does not have a selector, which means the style will apply to the children widgets of the main window all the way down the hierarchy. To change that, let's try something different:

QPushButton
{
  border: 2px solid gray;
  border-radius: 10px;
  padding: 0 8px;
  background: yellow;
}

This time, only the push button will get the style described in the preceding code, and all other widgets will return to the default styling. You can try to add a few more push buttons to your UI and they will all look the same:

This happens because we specifically tell the selector to apply the style to all the widgets with the class called QPushButton. We can also apply the style to just one of the push buttons by mentioning its name in the style sheet, like so:

QPushButton#pushButton_3
{
  border: 2px solid gray;
  border-radius: 10px;
  padding: 0 8px;
  background: yellow;
}

Once you understand this method, we can add the following code to the style sheet :

QPushButton
{
  color: red;
  border: 0px;
  padding: 0 8px;
  background: white;
}

QPushButton#pushButton_2
{
  border: 1px solid red;
  border-radius: 10px;
}

QPushButton#pushButton_3
{
  border: 2px solid gray;
  border-radius: 10px;
  padding: 0 8px;
  background: yellow;
}

What it does is basically change the style of all the push buttons as well as some properties of a specific button named pushButton_2. We keep the style sheet of pushButton_3 as it is. Now the buttons will look like this:

The first set of style sheet will change all widgets of QPushButton type to a white rectangular button with no border and red text. Then the second set of style sheet changes only the border of a specific QPushButton widget called pushButton_2. Notice that the background color and text color of pushButton_2 remain white and red respectively because we didn't override them in the second set of style sheet, hence it will return to the style described in the first set of style sheet since it's applicable to all QPushButton widgets. Do notice that the text of the third button has also changed to red because we didn't describe the color property in the third set of style sheet.

After that, create another set of style using the universal selector, like so:

*
{
  background: qradialgradient(cx: 0.3, cy: -0.4, fx: 0.3, fy: -0.4, radius: 1.35, stop: 0 #fff, stop: 1 #888);
  color: rgb(255, 255, 255);
  border: 1px solid #ffffff;
}

The universal selector will affect all the widgets regardless of their type. Therefore, the preceding style sheet will apply a nice gradient color to all the widgets' background as well as setting their text as white and giving them a one-pixel solid outline which is also in white. Instead of writing the name of the color (that is, white), we can also use the rgb function (rgb(255, 255, 255)) or hex code (#ffffff) to describe the color value.

Just as before, the preceding style sheet will not affect the push buttons because we have already given them their own styles which will override the general style described in the universal selector. Just remember that in Qt, the style that is more specific will ultimately be used when there is more than one style having influence on a widget. This is how the UI will look now: