This statement makes it sound like every Qt application uses CSS. That's not true. CSS is offered as a way to apply corporate branding to an application. I'm not aware of any FOSS Qt applications using CSS.
The applications that are ok with the default style don't need to specify style. Nevertheless Qt is an UI toolkit that supports CSS styling.
> I'm not aware of any FOSS Qt applications using CSS.
And that's ok. Nevertheless I've already worked on a couple of Qt projects that used Qt Style Sheets extensively. In fact, that's pretty much the norm in any non-desktop project.