How to get access and change style of an element with JavaScript?

window.onload = function() {
  const option = document.getElementsByClassName("button");

  const forward = document.getElementById("forward");

  Array.from(option).forEach(function(option) {
    option.addEventListener("click", () => {
      let keySelector = option.querySelector(".key-selector");
      if (option.classList.contains("selected-button")) {
        let selectedButton = option;
        //if selected element is clicked again turn the style-settings to default
        keySelector.style.color = "rgb(51, 51, 51)";
        keySelector.style.opacity = "0.6";

        selectedButton.style.backgroundColor = "rgb(226, 226, 226)";
        selectedButton.style.color = "black";
        selectedButton.style.opacity = "1";
        //return the colors when a button is deselected

        selectedButton.classList.remove("selected-button");
        console.log(option.className);
        //turn the class name to default to have provide the possibility to select the element again
      } else {
        option.style.backgroundColor = "rgb(77, 55, 120)";
        option.style.opacity = "0.65";
        option.style.color = "white";

        //Changong buttons backgroundcolor to purple

        keySelector.style.color = "white";
        //determine the key-selector and turn the color into white

        forward.style.color = "white";
        forward.style.backgroundColor = "rgb(77, 55, 120)";
        forward.style.transition = "1s ease";
        //changing the color of the continue-button to purple when at least one element is selected

        option.classList.add("selected-button");
        console.log(option.className);

        //replace class name of selected-element to provide the possibility to have access to the selected element in order to deselect if required
      }
    });
  });
};

body {
  font-family: 'Noto Sans Avestan', sans-serif;
}

.navbar {
  display: flex;
  list-style: none;
  background-color: rgb(77, 55, 120);
  margin: 0;
  position: fixed;
  width: 100%;
  gap: 4rem;
  height: 50px;
  text-align: center;
  line-height: 45px;
  left: 0;
  top: 0;
}

.nav-text {
  text-decoration: none;
  color: white;
  width: auto;
  cursor: pointer;
  font-size: 18px;
  padding-bottom: 5px;
}

.instruction {
  padding-top: 8rem;
  left: 10%;
  padding-bottom: 0px;
  width: auto;
  max-width: 730px;
  font-size: 20px;
  position: absolute;
}

.options {
  height: auto;
  max-height: 313px;
  max-width: 750px;
  width: auto;
  padding-top: 15rem;
  padding-bottom: 60px;
  display: flex;
  flex-direction: column;
  gap: 15px;
  position: sticky;
  left: 8rem;
}

.button {
  background-color: rgb(226, 226, 226);
  height: 418.75%;
  width: auto;
  padding: 21px 25px 22px 25px;
  box-sizing: border-box;
  border-top-left-radius: 5px;
  border-top-right-radius: 5px;
  border-bottom-left-radius: 5px;
  border-bottom-right-radius: 5px;
  cursor: pointer;
  font-size: 18px;
  line-height: 16.8px;
  display: block;
  position: relative;
  top: 0px;
  bottom: 0px;
  right: 0px;
  left: 0px;
}

.text {
  margin-left: 4rem;
}

.button:hover {
  background-color: rgb(194, 194, 194);
  opacity: 0.8;
}

#backward:hover,
#forward:hover {
  background-color: rgb(77, 55, 120);
  color: white;
}

.key-selector {
  position: absolute;
  top: 50%;
  margin-top: -12px;
  font-size: 16px;
  line-height: 1.5em;
  text-align: center;
  width: 30px;
  display: block;
  opacity: 0.6;
  border: 1px solid;
  border-radius: 5px;
  border-top-left-radius: 5px;
  border-top-right-radius: 5px;
  border-bottom-left-radius: 5px;
  border-bottom-right-radius: 5px;
  height: 25px;
  color: rgb(51, 51, 51);
}

.button:hover .key-selector {
  color: black;
}

.button-bar {
  position: fixed;
  bottom: 0;
  width: 100%;
  display: flex;
  margin: 0;
  left: 0;
}

.nav-inner {
  cursor: pointer;
  width: 50%;
  text-align: center;
  line-height: 83px;
}

#backward {
  background-color: rgb(101, 93, 93);
  color: white;
}

#forward {
  background-color: rgb(191, 191, 191);
}

<div id="work-container">
  <ul class="navbar">
    <li class="section"><a class="nav-text" href="client.html">A</a></li>
    <li class="section"><a class="nav-text" href="case.html" style=" border-bottom: 1px solid;">B</a></li>
  </ul>
  <div class="instruction">
    <h3>Please choose an answer. You can choose more than one answers.</h3>
  </div>
  <div class="options">
    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>A</span>
        </div>
        <div class="text">1</div>
      </div>

    </div>

    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>B</span>
        </div>
        <div class="text">2</div>
      </div>


    </div>

    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>C</span>
        </div>
        <div class="text">3</div>
      </div>

    </div>

    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>D</span>
        </div>
        <div class="text">4</div>
      </div>

    </div>

    <div class="option">
      <div class="button">
        <div class="key-selector">
          <span>E</span>
        </div>
        <div class="text">5</div>
      </div>

    </div>
  </div>


  <div class="button-bar">
    <div class="nav-inner" id="backward">
      < Back</div>
        <div class="nav-inner" id="forward"> Continue ></div>
    </div>
  </div>

</div>

button {
  color: black;
  background: aqua;
  border: none;
  border-radius: 5px;
  padding: 4px 5px;
  transition-duration: 0.2s;
}

button:disabled {
  background: silver;
  color: gray;
}

button:focus {
  background: #aaf;
  outline: 2px solid aqua;
}

<button disabled>Disabled</button>
<button>Back</button>
<button>Next</butotn>

<form>
  <input type="number" placeholder="Number"><br>
  <input type="password" placeholder="Password"><br>
  <input type="text" placeholder="Required text" required><br>
  <input type="date"><br>
  <input type="submit" value="Send"><br>
</form>