Qt Creator is the IDE (integrated development environment) used by Qt SDK. It is a powerful piece of software that runs on Linux, Windows and macOS. Qt SDK is able to style built applications using QSS styles, something similar to CSS standard.
Qt Creator supports not only C/C++ syntax highlighting, but basically any language through Kate Editor Syntax Highlighting files. These are XML files that define lists of specific keywords. In this way, the Text Editor of Qt Creator is able to style these keywords using a color scheme.
Qt Creator doesn't have a default QSS syntax highlight configuration file. And neither Kate. But there are two ways to add QSS syntax highlighting to Qt Creator.