Although having some experience with leaflet before, it took me 5 hours to find out how to
(1) change the background color and
(2) re-center my map after the popup was panning the map basically out view. Here is my solution
leaflet(options = leafletOptions( zoomControl = FALSE, minZoom=6, maxZoom=6, centerFixed = TRUE) ) %>% addPolygons(data = ds, fillColor = ~pal(AnzahlFall), color="black", weight = 1, fillOpacity = 0.7, label = ~name_2, popup = ~www, popupOptions = popupOptions( autopan=FALSE, keepInView = TRUE ) ) %>% htmlwidgets::onRender(" function(el, x) { var myMap = this; myMap.dragging.disable(); myMap.on('popupclose', function(e) { myMap.panTo( {lon: 10.26, lat: 51.1} ) }); var e = document.getElementsByClassName('leaflet-container'); e[0].style.backgroundColor = 'white'; }")