Qt Designer is an excellent tool to preview style sheets. You can right-click on any widget in Designer and select Change styleSheet... to set the style sheet.
In Qt 4.2 and later, Qt Designer also includes a style sheet syntax highlighter and validator. The validator indicates if the syntax is valid or invalid, at the bottom left of the Edit Style Sheet dialog.
When you click OK or Apply, Qt Designer will automatically display the widget with its new stylesheet.
