Solution: TrailRunner supports the following OSM Render Styles: Classic OpenStreetMap, OpenCycleMap and CloudMade Fine line. To switch between these, Goto to Bottom Toolbar > Background Map > Display Settings. In the left pane select any of the available OSM tile servers.
Tip: As tiles are heavily cached in TrailRunner, you may want to right click the map and reload background maps whenever you see outdated tiles being displayed from the cache. To reload all osm map tiles, delete the following folder: < yourhome >/Library/Caches/TrailRunner/MapViewTiles/openstreetmap.org