First off, thanks for your time.
I've tried that yeah - you can see that I've made the default index_file "qrc:/index-light.html" in the load_args function. It's here in the code.
When writing to qDebug() the string is correctly "qrc:/index-light.html", though nothing is shown in the webview... I've tried changing it to "qrc:/light.css" and then it correctly shows the css in the webview (the code in plaintext).