How can I apply a custom theme?

I am using QtCreator to create a Qt Application in C++.

I know CSS and making themes for elements in my applications isn't too hard, but is there a way to make a file and apply it?

I've looked through the Qt Docs but I can't seem to find anything about such a thing.

Currently, I am styling each individual button and stuff but can I just put it all in a file and apply it to everything at once?

Create a file, e.g., style.myStyle. There, you place the styles for all the widgets including events, attributes, etc. Then, load the file when the app starts, and apply that to the app:

#include <QApplication>
#include <QFile>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QFile file("./style.myStyle");
    file.open(QFile::ReadOnly);
    QString styleSheet = QLatin1String(file.readAll());
    a.setStyleSheet(styleSheet);
    MainWindow w;
    w.show();
    return a.exec();
}

Now in the style.myStyle file, you can do customize your style, e.g:

QPushButton
{
    background-color: white;
    border-style: outset;
    border-width: 2px;
    border-radius: 10px;
    border-color: beige;
    font: bold 14px;
    min-width: 10em;
    min-height: 20em;
    padding: 6px;
}
QPushButton:pressed
{
    background-color: rgb(224, 0, 0);
    border-style: inset;
}
QFrame, QLabel, QToolTip
{
    border: 2px solid green;
    border-radius: 2px;
    padding: 1px;
}

That code produces this:

Note: Make sure to validate that the file exists, etc.