Add coloration behind clusters

[mbjackson-capp]

The "number of points in this cluster" numbers on the map don't currently have any coloration behind them. The default behavior for a MarkerCluster is to have coloration; we should figure out how to restore that (and, perhaps, change coloration dynamically so smaller clusters have less intense color as you zoom in further)

The example image from Leaflet is
(which there is now a copy of in our repo)

[mbjackson-capp]

It looks like there are two basic ways to do this:

1. Change global settings in styles.css for entire app by adding CSS selectors for MarkerCluster
   https://stackoverflow.com/questions/49538465/changing-leaflet-markercluster-icon-color-inheriting-the-rest-of-the-default-cs

2. Explicitly set style when declaring the markerClusterGroup, as in:

var markers = L.markerClusterGroup({
    // Other options...
    styles: [{
        // Customize the cluster icon style
        textColor: '#fff',  // Text color
        iconSize: [30, 30],  // Icon size
        backgroundColor: '#007bff'  // Background color
    }]
});

(code suggestion from ChatGPT 3.5)

Unfortunately, in early attempts to apply each of these, neither of them worked. The additions to styles.css didn't even work when flagged as !important. Figure out what's going on here

[mbjackson-capp]

consider map.css