php-flarum/less/lib/avatars.less

.avatar-size(@size) {
  width: @size;
  height: @size;
  border-radius: @size;
  font-size: @size / 2;
  line-height: @size;
}
.avatar {
  display: inline-block;
  box-sizing: content-box;
  color: @fl-body-bg;
  font-weight: 300;
  text-align: center;
  vertical-align: top;
  background-color: @fl-body-control-bg;
  .avatar-size(48px);

  & img {
    display: inline-block;
    width: 100%;
    height: 100%;
    border-radius: 100%;
    vertical-align: top;
  }
}