純粋な CSS を使用して HTML を変更するにはどうすればよいですか?ラジオ/チェックボックスのデフォルトの背景色のスタイル

input[type=checkbox] {
  margin-right: 5px;
  cursor: pointer;
  font-size: 14px;
  width: 15px;
  height: 12px;
  position: relative;
}
input[type=checkbox]:after {
  position: absolute;
  width: 10px;
  height: 15px;
  top: 0;
  content: " ";
  background-color: #ff0000;
  color: #fff;
  display: inline-block;
  visibility: visible;
  padding: 0px 3px;
  border-radius: 3px;
}
input[type=checkbox]:checked:after {
  content: "✓";
  font-size: 12px;
}
input[type=radio] {
    margin-right: 5px;
    cursor: pointer;
    font-size: 14px;
    width: 15px;
    height: 12px;
    position: relative;
}
input[type=radio]:after {
    position: absolute;
    width: 10px;
    height: 15px;
    top: 0;
    content: " ";
    background-color: #ff0000;
    color: #fff;
    display: inline-block;
    visibility: visible;
    padding: 0px 3px;
    border-radius: 50%;
}
input[type=radio]:checked:before {
    content: "✓";
    display: block;
    position: relative;
    top: -2px;
    left: 3px;
    width: 6px;
    height: 6px;
    color: #fff;
    font-weight: 400;
    z-index: 1;
}

<input type="checkbox" name="method" value="plus" checked>plus<br>
<input type="checkbox" name="method" value="minus">minus<br>
<input type="radio" name="method" value="plus">plus<br>
<input type="radio" name="method" value="minus" checked>minus<br>