As a viewer of Marble's vector map I'd like to be aware of streets that I cannot enter because they have restricted access (e.g. private roads). I'd like to see those streets marked with a different color.

The commonly used tags for describing access restrictions are described in https://wiki.openstreetmap.org/wiki/Key:access
For now I'd suggest to use the key 'access' and the following values to consider a street as restricted: private, no, agricultural, delivery, forestry.

To implement this, change OsmWay.cpp around line 109 where one way streets and lane width are already covered. The style change can be done similarly to the handling of tunnels in that section.

When T768 and/or T769 are implemented by @rahn we can switch the styling to a similar one also: Use a dot decorator effect instead of changing the whole color. This is the effect mapnik uses as well, see e.g. https://www.openstreetmap.org/#map=19/49.00710/8.40437 (street to the parking lot in the center).