Style tweaks for Leaflet 0.7

This update brings the locate control in line with 0.7.1, which introduced a more subtle border around mobile controls. See https://github.com/Leaflet/Leaflet/issues/1868 and https://github.com/Leaflet/Leaflet/commit/ea6095f259a1d3ba950526e446d4f8d11334abae.