If you want to save the current QStyle of your Application to the Settings you have to use something like this:

 
QStyle *style = qApp->style();

QString styleKey;

if(dynamic_cast<MyStyle*>(style) != NULL)
    styleKey = "MyStyle";
else if(dynamic_cast< QPlastiqueStyle*>(style) != NULL)
    styleKey = "Plastique";    
else if(dynamic_cast<QWindowsXPStyle*>(style) != NULL)
    styleKey = "WindowsXP";
[...]

settings.setValue("WidgetStyle",styleKey);

It would be nice to have a member function (eg. virtual QString key()) which will return the key identical to the QStyleFactor::keys() function.
So you only have to call QString styleKey = qApp->style()->key(); to get the key.

And simply with QStyleFactor::create(key) you can restore