This is possible with 04a3d5e881368111983003ba32d29e1871eebc80. There's now only a single place to define stylesheet information in the application, and the user can override the builtin style.

The only thing remaining is to install a default stylesheet and use that one instead of the hardcoded one in main.cpp.
The API for QWidget::setStylesheet() is not tailored towards reading a file, though, so I will call it a day and leave it as is for now.