Setting style sheet of ui in main window class from dialog class

[anda_skoa]

You don't say what your problem is, so I am guessing: move the signal/slot connect before the call to exec()

Dialog *d = new Dialog;
connect(d, SIGNAL(update(QString)),this,SLOT(Set_style_Sheet(QString)));
d->exec();

Dialog *d = new Dialog;
connect(d, SIGNAL(update(QString)),this,SLOT(Set_style_Sheet(QString)));
d->exec();

To copy to clipboard, switch view to plain text mode 

Btw, creating a dialog with new and not deleting it results in memory loss.

Cheers,
_