GeoJson map background color within leaflet

Hello R users!

I'm using a geoJson file to produce a map with leaflet. I would like to change the background color, or - at least - change the color of the parts without land. When we zoom the countries borders confound themselves it the sea. The example we produced:

I see that if I run code like this to the geoJson object I able to modify some maps, but not all of them:

geoJson$style = list(
  weight = 1,
  color = "#555555",
  opacity = 1,
  fillColor = 'green',
  fillOpacity = 1)