Styling the expand row button in DT in shiny R

I'm trying to style the expand rows button available in DT. The styling is available here. The code I'm using for creating the datatable is-

library(DT)
datatable(
  cbind(' ' = '&oplus;', mtcars), escape = -2,
  options = list(
    columnDefs = list(
      list(visible = FALSE, targets = c(0, 2, 3)),
      list(orderable = FALSE, className = 'details-control', targets = 1)
    )
  ),
  callback = JS("
  table.column(1).nodes().to$().css({cursor: 'pointer'});
  var format = function(d) {
    return '<div style=\"background-color:#eee; padding: .5em;\"> Model: ' +
            d[0] + ', mpg: ' + d[2] + ', cyl: ' + d[3] + '</div>';
  };
  table.on('click', 'td.details-control', function() {
    var td = $(this), row = table.row(td.closest('tr'));
    if (row.child.isShown()) {
      row.child.hide();
      td.html('&oplus;');
    } else {
      row.child(format(row.data())).show();
      td.html('&CircleMinus;');
    }
  });"
))*

I'm not well versed with JS so I'm lost where to start off. I've tried using the JS and CSS files from the link but haven't been able to make any progress. Any leads appreciated, thanks!

This works like this if you open the table in the browser (this doesn't work in the RStudio viewer). But I suspect there's an easier solution... Moreover this solution relies on images hosted on Github, that's not very cool...

library(DT)
datatable(
  cbind(' ' = '<img src=\"https://raw.githubusercontent.com/DataTables/DataTables/master/examples/resources/details_open.png\"/>', 
        mtcars), escape = -2,
  options = list(
    columnDefs = list(
      list(visible = FALSE, targets = c(0, 2, 3)),
      list(orderable = FALSE, className = 'details-control', targets = 1)
    )
  ),
  callback = JS("
                table.column(1).nodes().to$().css({cursor: 'pointer'});
                var format = function(d) {
                return '<div style=\"background-color:#eee; padding: .5em;\"> Model: ' +
                d[0] + ', mpg: ' + d[2] + ', cyl: ' + d[3] + '</div>';
                };
                table.on('click', 'td.details-control', function() {
                var td = $(this), row = table.row(td.closest('tr'));
                if (row.child.isShown()) {
                row.child.hide();
                td.html('<img src=\"https://raw.githubusercontent.com/DataTables/DataTables/master/examples/resources/details_open.png\"/>');
                } else {
                row.child(format(row.data())).show();
                td.html('<img src=\"https://raw.githubusercontent.com/DataTables/DataTables/master/examples/resources/details_close.png\"/>');
                }
                });"
))