Create Choropleth coloring county_outline parameter

I’m having trouble affecting the “county_outline” parameter of the create_choropleth function from the figure factory library.

I want to be able to customize the county outline color and width without affecting all counties on the map, such as what is done on this graph which I would like to be able to replicate

But I can’t seem to affect anything smaller than the entire set of counties; see this map where I set the width to 1 and the color to black.

I took my code from this example of county_choropleth from plotly
I’ve tried setting county_outline to an array of colors, but that doesn’t change anything. Anyone got ideas?