:focus-within

label {
  display: block;
  margin-top: 1em;
}
label:focus-within {
  font-weight: bold;
}

<form>
  <p>Which flavor would you like to order?</p>
  <label>Full Name: <input name="firstName" type="text" /></label>
  <label
    >Flavor:
    <select name="flavor">
      <option>Cherry</option>
      <option>Green Tea</option>
      <option>Moose Tracks</option>
      <option>Mint Chip</option>
    </select>
  </label>
</form>

:focus-within {
  /* ... */
}

<p>このフォームに入力してみてください。</p>
<form>
  <label for="given_name">名前:</label>
  <input id="given_name" type="text" />
  <br />
  <label for="family_name">苗字:</label>
  <input id="family_name" type="text" />
</form>

form {
  border: 1px solid;
  color: gray;
  padding: 4px;
}
form:focus-within {
  background: #ff8;
  color: black;
}
input {
  margin: 4px;
}