As far as I understand, QtWidgets can be styled via CSS using the method setStyleSheet.
See reference: QtWidgets styleSheet
The problem is I couldn't figure out if it supports CSS3 or since which version of Qt CSS3 capabilities are supported and to which extend.
Any clarifying information highly appreciated!
Qt stylesheets are not standardized. Not all CSS3 properties are supported by Qt, and some of Qt-supported properties aren't supported by regular browsers. You should look at Qt Style Sheets Reference and other docs to know supported features list.