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Do you really want to use the mapbox basic style? Otherwise you can set style=‘carto-positron’ which doesn’t require access token.
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The distribution of color over counties is nonuniform because only one county has assigned a big value mapped to the colorscale. Other counties have their corresponding values close to minimum.
To remedy this color distribution you can use a discrete colorscale. I explained here https://community.plotly.com/t/add-geojson-polygons-seaborn-colored-by-specific-feature/40464/2 how it is defined. Choose a number of distinct colors (not necessarily colors in a seaborn palette) and divide the interval of values in a few subintervals.