Those screens are meant for manual focus lenses... the matt part will screen out the image while the clear part allows u to focus on the subject. As u turn the focus ring, the image will sharpen and the effect of the 'infocus' look will be enhanced making it easier to find out if the image is in focus.
As for your focussing problem... I think in that pic u posted, the girl is the target right? The problem here is that the 7D's AF is not very good nor clever. The centre focus sensor (that centre box in your viewfinder) does not represent the sensor actually... if u take a good look at your manual, u will find that the actual sensor physically occupies only a portion of the centre focus spot in the viewfinder... under poor lighting, the sensor may get fooled and goes for the highest contrast object available (or what it deems to be highest contrast)... that's probably why the focus is always wrong...
On the other hand, it might be a calibration problem with your AF sensors - it's reading the lens wrongly... u can try with another similar scene but in daylight and see if the same thing occurs...
If u want to test, can try moving the subject closer to u while u keep the same background perspective.