geostyle.css

#museums{
  marker-fill: red;
}

mapconfig.json

{
    "title": "Bergamo Example",
    "map": {
        "center": [
            9.634,
            45.7
        ],
        "centerProjection": "EPSG:4326",
        "zoom": 12,
        "minZoom": 10,
        "maxZoom": 17,
        "extent": [
            9,
            45.2,
            10,
            46
        ],
        "extentProjection": "EPSG:4326"
    },
    "layers": [
        {
            "layerType": "stamen",
            "layerOptions": {
                "opacity": 1,
                "layer": "watercolor"
            },
            "name": "layer3"
        },
        {
            "name": "darkmatter",
            "layerType": "cartodb",
            "layerOptions": {
                "layer": "dark-matter",
                "opacity": "0",
                "visible": false
            }
        },
        {
            "name": "museums",
            "layerType": "geojson",
            "layerOptions": {
                "url": "https://cdn.rawgit.com/inmagik/html-map-bg/master/vectors/museum_wgs.geojson",
                "opacity": 1
            },
            "templatePopup": "templates/table.html"
        }
    ]
}