main.css

*{
    text-align: center;
    letter-spacing: 1px;
       font-family: 'Avenir','Helvetica Neue','Helvetica','Arial','Hiragino Sans','ヒラギノ角ゴシック',YuGothic,'Yu Gothic','メイリオ', Meiryo,'ＭＳ Ｐゴシック','MS PGothic' ;
}

body>h1{
    font-size: 36px;
    margin: 70px 10%;
    letter-spacing: 3px;
}

body>h1::after{
    content: "";
    display: block;
    height: 3px;  width: 10%;
    margin: 0 auto;
    background: skyblue;
    margin-top: 50px;
}

.Introduction p{
    margin: 30px 10%;
    font-size: 18px;
    font-weight: bold;
}

.Introduction a{
    display: inline-block;
    width: 30%;
    border: solid 5px rgb(255,181,170);
    font-size: 16px;
    font-weight: bold;
    line-height: 36px;
    border-radius: 5px;
    text-decoration: none;
    color: black;
    transition: all .3s;
}

.Introduction a:hover{
    color: white;
    background: rgb(255,181,170);
}

.Container>div{
    display: flex;
    justify-content: center;
    align-content: flex-start;
    flex-wrap: wrap;
    margin: 5% 10%;
    padding-top: 30px;
    border-top: solid 1px #CCC;
}

.Container>div li{
    list-style: none;
    text-align: left;
    width: calc(50% - 31px);
    margin: 15px;
    border: solid 4px skyblue;
    box-sizing: border-box;
    font-size: 16px;
    font-weight: bold;
    line-height: 36px;
    border-radius: 5px;
}

.Container>div h2{
    width: 100%;
    font-size: 32px;
    padding: 5%;
}

.Container>div h2::after{
    content: "";
    display: block;
    height: 3px;  width: 10%;
    margin: 0 auto;
    background: skyblue;
    margin-top: 50px;
}

.Container>div::after{
    content: "";
    display: inline-block;
    width: 50%;
}

.Container>div li a{
    text-decoration: none;
    display: inline-block;
    color: black;
    height: calc(100% - 20px);
    width: calc(100% - 20px);
    padding: 10px;
    transition: all .3s;
}

.Container>div li:hover{
    background: skyblue;
}

.Container>div li:hover a{
    color: white;
}

.Container>div li a:after{
    content: "";
    width: 100%; height: 100%;
}

@media screen and (max-width: 720px){
    .Introduction a{
        width: calc(80% - 30px);
    }
    .Container>div li{
        width: calc(100% - 30px);
    }
}