WPF and MahApps

I am creating a component which opens a WPF window and I wish to configure it with MahApps following the indicated instructions (MahApps.Metro - Quick Start). Everything worked well before making the indicated changes to integrate MahApps. I have randomly chosen Light.Blue as Theme at the <Application.Resources> but the poping window turns black (photos bellow), which makes me think that MahApps is not really recognized/linked. Any help/idea would be much appreciated!

Without MahApps;

With MahApps: