* {
    box-sizing: border-box;
  }

  body {
    margin: 0;
    font-family: "Hiragino Kaku Gothic ProN";
  }

  a {
    text-decoration: none;
  }

  .container {
    width: 100%;
    padding: 0 15px;
    margin: 0 auto;
  }

  .top-wrapper {
    padding: 180px 0 100px 0;
    background-image: url(top.png);
    background-size: cover;
    color: white;
    text-align: center;
  }

  .top-wrapper h1 {
    opacity: 0.9;
    font-size: 45px;
    letter-spacing: 5px;
  }

  .top-wrapper h2 {
    opacity: 0.85;
    font-size: 25px;
    letter-spacing: 5px;
  }

  .top-wrapper p {
    opacity: 0.7;
    font-size: 16px;
    margin-bottom: 40px;
  }

  .btn-wrapper {
    text-align: center;
  }

  .btn-wrapper p {
    margin-bottom: 20px;
  }

  .btn {
    padding: 8px 24px;
    color: white;
    display: inline-block;
    opacity: 0.8;
    border-radius: 4px;
    text-align: center;
  }

  .btn:hover {
    opacity: 1;
  }

  .fa {
    margin-right: 5px;
  }

  header {
    height: 65px;
    width: 100%;
    background-color: rgba(44, 63, 62, 0.9);
    position :fixed;
    top: 0;
    z-index: 10;
  }

  .logo {
    width: 124px;
    margin-top: 20px;
  }

  .header-left {
    float: left;
  }

  .header-left h2 {
    line-height: 25px;
    padding: 0 25px;
    color: white;
    float: left;
    display: block;
  }

  .header-left a {
    background-color: white;
    color: rgba(44, 63, 62, 0.9);
    line-height: 25px;
    padding: 15px 15px;
    border-radius: 50%;
    font-weight: bold;
    transition: 0.5s;
  }

  .header-left a:hover {
    background-color: rgba(196, 72, 111, 0.3);
    color: white;
  }

  .header-right {
    float: right;
    margin-right: -25px;
  }

  .header-right a {
    line-height: 65px;
    padding: 0 25px;
    color: white;
    display: block;
    float: left;
    transition: all 0.5s;
  }

  .header-right a:hover {
    background-color: rgba(196, 72, 111, 0.3);
  }

  .lesson-wrapper {
    height: 700px;
    padding-bottom: 80px;
    padding-left: 5%;
    padding-right: -5%;
    background-color: #f8f2f2;
    text-align: center;
  }

  .heading {
    padding-top: 30px;
    padding-bottom: 50px;
    color: #5f5d60;
  }

  .heading h1 {
    font-size: 30px;
  }

  .lesson {
    float: left;
    width: 50%;
  }

  .lesson-icon {
    position: relative;
  }

  .lesson-icon p {
    position: absolute;
    top: 37%;
    width: 100%;
    color: white;
  }

  .text-contents {
    width: 50%;
    display: inline-block;
    margin-top: 15px;
    font-size: 15px;
    color: #b3aeb5;
  }

  .heading h3 {
    font-weight: normal;
  }

  .message-wrapper {
    border-bottom: 1px solid #eee;
    padding-bottom: 80px;
    text-align: center;
  }

  .message {
    padding: 15px 40px;
    background-color: #5dca88;
    cursor: pointer;
    box-shadow: 0px 7px #1a7940;
  }

  .message:active {
    position: relative;
    top: 7px;
    box-shadow: none;
  }

  footer img {
    width: 125px;
  }

  footer p {
    color: #b3aeb5;
    font-size: 12px;
  }

  footer {
    padding-top: 30px;
    padding-bottom: 20px;
  }

  .menu-icon {
    color: white;
    float: right;
    font-size: 25px;
    padding: 21px 0;
    display: none;
  }

  .clear {
    clear: left;
  }

@media all and (max-width: 1000px) {
  .lesson {
    width: 50%;
    margin-bottom: 50px;
  }

  .lesson-wrapper {
    height: 750px;
  }

  footer {
    text-align: center;
  }
}

@media all and (max-width: 750px) {
  .top-wrapper h1 {
    font-size: 32px;
  }

  .heading h2 {
    font-size: 20px;
  }
}

@media all and (max-width: 670px) {
  .header-right {
    display: flex;
    flex-direction: column;
    background-color: rgba(44, 63, 62, 0.95);
    position: absolute;
    top: 65px;
    right: 0;
    width: 60%; /* 例えば60% */
    max-width: 300px;
    text-align: center;
    z-index: 9;
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.3s ease;
  }
  

  .header-right a {
    padding: 15px 0;
    border-top: 1px solid rgba(255, 255, 255, 0.1);
  }  

  .menu-icon {
    display: block;
  }

  .top-wrapper .btn {
    width: 100%;
  }

  .twitter {
    margin-top: 10px;
  }

  .top-wrapper {
    text-align: left;
  }

  .top-wrapper h1 {
    font-size: 24px;
  }

  .top-wrapper p {
    font-size: 14px;
  }

  .lesson {
    width: 100%;
  }

  .lesson-wrapper {
    height: 1300px;
  }

  .message-wrapper .btn {
    width: 100%;
  }
  .menu-icon {
    width: 30px;
    height: 25px;
    position: relative;
    cursor: pointer;
    padding: 20px 0;
  }
  
  .menu-icon span {
    background-color: white;
    height: 3px;
    width: 100%;
    position: absolute;
    left: 0;
    transition: all 0.3s;
    margin: 20px 0;
  }
  
  .menu-icon span:nth-child(1) {
    top: 0;
  }
  
  .menu-icon span:nth-child(2) {
    top: 11px;
  }
  
  .menu-icon span:nth-child(3) {
    top: 22px;
  }   

  .header-right.active {
    display: flex;
    flex-direction: column;
    background-color: rgba(44, 63, 62, 0.9);
    position: absolute;
    right: 0;
    top: 65px;
    width: 100%;
    display: flex;
    opacity: 1;
    visibility: visible;
  }

  .header-right.active a {
    padding: 15px 0;
    border-top: 1px solid #ccc;
    text-align: center;
  }
}