:disabled CSS pseudo-class

label {
  display: block;
  margin-top: 1em;
}

*:disabled {
  background-color: dimgrey;
  color: linen;
  opacity: 1;
}

<form>
  <label for="name">Name:</label>
  <input id="name" name="name" type="text">

  <label for="emp">Employed:</label>
  <select id="emp" name="emp" disabled="">
    <option>No</option>
    <option>Yes</option>
  </select>

  <label for="empDate">Employment Date:</label>
  <input id="empDate" name="empDate" type="date" disabled="">

  <label for="resume">Resume:</label>
  <input id="resume" name="resume" type="file">
</form>

:disabled {
  /* ... */
}

<form action="#">
  <fieldset id="shipping">
    <legend>Shipping address</legend>
    <input type="text" placeholder="Name" />
    <input type="text" placeholder="Address" />
    <input type="text" placeholder="Zip Code" />
  </fieldset>
  <br />
  <fieldset id="billing">
    <legend>Billing address</legend>
    <label for="billing-checkbox">Same as shipping address:</label>
    <input type="checkbox" id="billing-checkbox" checked />
    <br />
    <input type="text" placeholder="Name" disabled />
    <input type="text" placeholder="Address" disabled />
    <input type="text" placeholder="Zip Code" disabled />
  </fieldset>
</form>

input[type="text"]:disabled {
  background: #cccccc;
}

const checkbox = document.querySelector("#billing-checkbox");
const billingItems = document.querySelectorAll('#billing input[type="text"]');

checkbox.addEventListener("change", () => {
  billingItems.forEach((item) => {
    item.disabled = !item.disabled;
  });
});