style.css

body{
    margin: 0;
}

div {
    border: 0px;
}

 @media screen and (min-width:400px) and (min-height:650px){
    
    a {
        display: inline-block;
    }

    a.rm {
        position: absolute;
        right: 0;
    }

    hr {
        opacity: 0.2;
    }

    object{
        pointer-events: none;
        background-color: white;
        border-radius: 50px;
        margin: 0;
        transition: all linear 0.1s;
    }

    ul{
        list-style: none;
        padding-left: 0px;
        margin: 2px 5px 18px 5px;
    }

    li._todolist{
        padding-bottom: 18px;
        position: relative;
    }

    /* 진짜 체크박스 */
    input[type = "checkbox"] {
        width: 0;
        height: 0;
        padding: 0;
        margin: -1px;
        border: 0;
    }

    input[type="checkbox"] + label {
        display: inline-block;
        position: relative;
        cursor: pointer;
        -webkit-user-select: none;
        -moz-user-select: none;
        -ms-user-select: none;
    }

    input[type="checkbox"]:checked + label {
        color: #cacece;
    }

    /* 가짜 체크박스 */
    input[type="checkbox"] + label:before {
        content: ' ';
        display: inline-block;
        width: 21px;
        /* 체크박스의 너비를 지정 */
        height: 21px;
        /* 체크박스의 높이를 지정 */
        line-height: 25px;
        /* 세로정렬을 위해 높이값과 일치 */
        margin: -2px 8px 0 0;
        text-align: center;
        vertical-align: middle;
        background: #ffffff;
        border: 1px solid #cacece;
        border-radius : 15px;
        /* 출처: https: //webdir.tistory.com/433 [WEBDIR]*/
    }

    input[type="checkbox"]:checked + label:before {
        content: '\2713';
        /* 체크표시 유니코드 사용 */
        color: #04B486;
        border-color: #04B486;
        font-size: 20px;
        text-align: center;
    }

    /* 출처: https: //webdir.tistory.com/433 [WEBDIR]*/

    input[type="text"] {
        position: absolute;
        left: 10%;
        top: 35%;
        width: 80%;
        line-height: 2.5em;
    }

    .background{
        width: 100%;
        height: 100%;
        left: 0;
        top: 0;
        margin: 0;
        background-color: #ecedee;
        display: flex;
        align-items: center;
        justify-content: center;
    }

    .error{
        visibility: hidden;
        display: none;
    }

    .main-box{
        background-color: #ffffff;
        border-radius: 10px;
        position: absolute;
        height: 600px;
        width: 400px;
        overflow-y: hidden;
        visibility: visible;
    }

    .sub-box{
        padding-left: 7%;
        padding-right: 7%;
    }

    .external-box{
        background-color: #cacece;
        position: absolute;
        top:80%;
        width: 100%;
        height: 20%;
    }

    .toggle-wrapper {
        position: relative;
        top:300px;
    }

    #body{
        height: 70%;
        overflow: auto;
        margin: 20px 0px;
    }

    #tail{

    }

    #date{
        border-radius: inherit;
        padding-top: 20px;
    }

    h2#ymd{
        margin-bottom: 0px;
        color: #2E2E2E;
    }

    p#day {
        margin-top: 5px;
        color: gray;
    }

    #remainder {
        color: #04B486;
    }

    #remainder > b > p {
        padding-top: 15px;
    }

}

@media not screen and (min-width:400px) and (min-height:650px){
    .background{
        width: 100%;
        height: 100%;
        left: 0;
        top: 0;
        margin: 0;
        background-color: #FFFFFF;
        display: flex;
        align-items: center;
        justify-content: center;
    }

    .error{
        text-align: center;
        position: absolute;
    }

    .main-box{
        visibility: hidden;
    }
    
    .toggle-wrapper{
        visibility: hidden;
    }
}