Gist that contains all the material needed to start working on the React JS version of my Tic Tac Toe game

App.css

#game {
    background: var(--content-background-color);
    border-radius: var(--default-border-radius);
    padding: 3vmin;
    box-shadow: var(--box-shadow);
}

h1 {
    color: var(--primary-color);
    text-align: center;
    margin-bottom: 2vmin;
    font-size: var(--main-title-font-size);
}

#board {
    display: grid;
    grid-template-columns: repeat(3, 20vmin);
    grid-template-rows: repeat(3, 20vmin);
    grid-gap: 1vmin;
    margin: 2vmin 0;
}

.cell {
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 10vmin;
    font-weight: bold;
    color: var(--accent-color);
    background: var(--primary-gradient);
    border: var(--default-primary-border);
    border-radius: var(--default-border-radius);
    outline: none;
    box-shadow: var(--box-shadow);
}

.cell:active, .cell:hover, .cell.winner {
    cursor: pointer;
    color: var(--content-background-color);
    background: var(--primary-highlight-gradient);
}

.cell-content {
    overflow: hidden;
    width: 0;
    height: 0;
    transition: width var(--selection-animation-duration), height var(--selection-animation-duration);
}

.cell-content.populated {
    width: 12vmin;
    height: 12vmin;
}

#modal-overlay {
    display: none;
    justify-content: center;
    position: absolute;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
    background: var(--modal-overlay-color);
}

#game-result-modal {
    display: grid;
    grid-row-gap: 76vmin;

    --modal-items-border-radius: 1vmin;
}

#result-container {
    display: flex;
    align-items: flex-end;
}

#winner-container {
    margin-bottom: 4vmin;
    width: 30vmin;
    height: 8vmin;
    font-size: var(--main-title-font-size);
    font-weight: bold;
    color: var(--primary-color);
    background: var(--accent-color);
    border: var(--default-accent-border);
    box-shadow: var(--box-shadow);
    border-radius: var(--modal-items-border-radius);
    display: flex;
    justify-content: center;
    align-items: center;
}

#new-game-container {
    display: flex;
    justify-content: center;
    align-items: flex-start;
    padding-top: 2.5vmin;
}

#new-game-container > button {
    font-size: 3vmin;
    padding: 1.5vmin;
    color: var(--accent-color);
    background: var(--primary-gradient);
    border: var(--default-primary-border);
    border-radius: var(--modal-items-border-radius);
    outline: none;
    box-shadow: var(--box-shadow);
}

#new-game-container > button:hover,
#new-game-container > button:active {
    color: var(--content-background-color);
    cursor: pointer;
    background: var(--primary-highlight-gradient);
}

index.css

:root {
    --background-light-color: #57ebd7;
    --background-dark-color: #24a88b;
    --content-background-color: #fff6be;
    --primary-color: #007340;
    --primary-brighter: #00a057;
    --primary-saturated: #05a058;
    --primary-saturated-brighter: #07cf72;
    --accent-color: #FD0;
    --accent-darker: #a79000;
    --box-shadow-color: rgba(0,0,0,0.75);
    --modal-overlay-color: rgba(0, 0, 0, 0.3);

    --main-background-gradient: linear-gradient(to right, var(--background-light-color) 0%, var(--background-dark-color) 100%);
    --primary-gradient: linear-gradient(to right, var(--primary-color) 0%,var(--primary-saturated) 100%);
    --primary-highlight-gradient: linear-gradient(to right, var(--primary-brighter) 0%,var(--primary-saturated-brighter) 100%);
    --box-shadow: 0 1vmin 2vmin -1vmin rgba(0,0,0,0.75);
    --default-border-radius: 2vmin;
    --default-primary-border: 0.25vmin solid var(--primary-color);
    --default-accent-border: 0.25vmin solid var(--accent-darker);
    --main-title-font-size: 4vmin;

    --selection-animation-duration: 350ms;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html, body {
    height: 100%;
}

body {
    display: grid;
    place-items: center center;
    background: var(--main-background-gradient);
    font-family: Arial, Helvetica, sans-serif;
}

index.html

<div id="game">
      <h1>Tic Tac Toe</h1>
      <div id="board">
          <button class="cell">
              <span class="cell-content"></span>
          </button>
          <button class="cell">
              <span class="cell-content"></span>
          </button>
          <button class="cell">
              <span class="cell-content"></span>
          </button>
          <button class="cell">
              <span class="cell-content"></span>
          </button>
          <button class="cell">
              <span class="cell-content"></span>
          </button>
          <button class="cell">
              <span class="cell-content"></span>
          </button>
          <button class="cell">
              <span class="cell-content"></span>
          </button>
          <button class="cell">
              <span class="cell-content"></span>
          </button>
          <button class="cell">
              <span class="cell-content"></span>
          </button>
      </div>
  </div>

  <div id="modal-overlay">
      <div id="game-result-modal">
          <div id="result-container">
              <div id="winner-container">
                  <span></span>
              </div>
          </div>
          <div id="new-game-container">
              <button id="new-game-button">Start New Game</button>
          </div>
      </div>
  </div>

matrix.js

const winningMatrix = {
    0: [[1,2],[3,6],[4,8]],
    1: [[0,2],[4,7]],
    2: [[0,1],[5,8],[4,6]],
    3: [[0,6],[4,5]],
    4: [[2,6],[3,5],[1,7],[0,8]],
    5: [[3,4],[2,8]],
    6: [[7,8],[0,3],[2,4]],
    7: [[6,8],[1,4]],
    8: [[6,7],[2,5],[0,4]]
};