Recently I had the trouble of changing the default radio checkbox in ionic. Here is the CSS of how to do this:

.list .item-radio .item-content {
  background-color: black;
  color: white;
}
.list .item-radio .radio-icon {
  background-color: black;
  color: white;
}
.item-radio input:checked + .radio-content .item-content {
    /* style the item content when its checked */
    background: black; 
}