Hi everyone,
I got the answer. If you have a number of buttons with different features then you can use a single qss file to maintain the style sheet for all those. For that you have to use the following code;

QPushButton[text = 'OK'] {
color: green;
}

QPushButton[text = 'Cancel'] {
color: blue;
}
Here in place of text you can use any unique feature or you can use the name of the object to show them in different styles.

Hope this will help others.