style.css

*{
    margin: 0;
    padding: 0;
    font-family: 'Roboto', sans-serif;   
}
header{
    width: 100%;
    height: 100px;
    background: #2b3349;
    padding: 20px;
    box-sizing: border-box;
}
header>h1,h4{
    color: rgba(255, 255, 255, .9);
    font-weight: 300;
}

section{
    width: 100%;
    height: 100%;
    background: rgba(30, 41, 65, 0.9);
    position: fixed;
    padding: 20px;
    box-sizing: border-box;
}

article{
    width: 100%;
    position: relative;
    float: left;
}

    article>input{
        width: calc(100% - 60px);
        height: 40px;
        border-radius: 25px;
        border: none;
        padding: 0px 15px;
        box-sizing: border-box;
        outline: none;
        background: rgba(105, 110, 122, 0.4);
        color: rgba(255, 255, 255, .8);
    }
    article>input::placeholder{
        color: rgba(255, 255, 255, 0.959);
    }
    article>button{
        width: 40px;
        height: 40px;
        border-radius: 50%;
        color: white;
        background: #2479A9;
        border: none;
        outline: none;
        font-size: 22px;
        font-weight: 500;
        margin-left: 10px;
    }
 section>h3{
    color: rgba(255, 255, 255, .9);
    font-weight: 300;
 }  
 label{
    margin-left: 20px;
    color: rgba(255, 255, 255, .9);
    font-weight: 300;
 } 
 .terminado{
    color: red;
    text-decoration:line-through;
    font: bold 90% monospace;
 }

.tarea{
    margin-top: 20px;
    border-radius: 50px;
    background: rgba(105, 110, 122, 0.4);
    width: 95%;
    height: 60px;
    display: flex;
    flex-direction: row;
    margin-bottom: 15px;
    display: flex;
    justify-content: start;
    padding-left: 20px;
    align-items: center;
}
input[type='checkbox'] {
    appearance: none;
    height: 20px;    
    width: 20px;
    opacity: 35%;
    border: white solid 1px;
}