<div class="paper border shadow shadow-large shadow-hover">
  <form>  
    <div>
      <label for="input">Target label as the previous sibling</label>
      <input id="input" type="checkbox">
    </div>
    <div>
      <label for="input1">
        <input id="input1" type="checkbox">
        Target label as the parent element
      </label>      
    </div>
  </form>
</div>

@import url("https://unpkg.com/papercss@1.9.2/dist/paper.css") layer(csspaper);

label:has(+ input:checked) {
  color: green;
  text-decoration: line-through;
}

label:has(input:checked) {
  color: green;
  text-decoration: line-through;
}

/* Demo style */
body {
  display: grid;
  place-items: center;
  min-height: 100vh;
  background-color: #f6eee3;
  background-size: 20px 20px;
  background-image:  repeating-linear-gradient(0deg, #e5decf, #e5decf 1px, #f6eee3 1px, #f6eee3);
}

form div {
  display: flex;
  gap: 10px;
  flex-direction: row-reverse;
  margin-block: 1rem;
}

label {
  display: flex;
  gap: 10px;
}