How to apply an existing symbol in pyQGIS?

Question

I'd like to change the layer rendering style from python code. Basically I want to do the same operations I can do from QGis by:

1. Right-click on a layer
2. Select Properties
3. Select one of the displayed Saved styles, and Apply changes

I found several examples on the net, but all talks about loading custom styles from files. I don't want (at least for the moment) do that. I need to use already available styles provided from QGis application.

I have a style called 'Construction road' on the property menu, so I tried the following:

aLayer = self.osmNavigator.iface.addVectorLayer(uri.uri(),layerName,self.dbConn.getProviderName())
aLayer.loadNamedStyle("Construction road")

but this doesn't work.

My question are:

1. How can I change the rendering style of a QGisVectorLayer from a python plugin, selecting one from already available styles?
2. Can I retrieve a list of all available style names?