*{
    font-family: sans-serif;
}
body{
    background-color: rgb(30, 30, 30);
    display: flex;
    justify-content: center;
    align-items: center;
    margin-top: 25%;
}
button{
    position: relative;
    width: 200px;
    padding: 15px 0;
    text-align: center;
    margin: 20px 10px;
    font-weight: bold;
    border: 2px solid #3eff37;
    background: transparent;
    color: #fff;
    cursor: pointer;
    overflow: hidden;
    text-transform: uppercase;
}
span{
    background: #3eff37;
    height: 100%;
    width: 0;
    position: absolute;
    left: 0;
    bottom: 0;
    z-index: -1;
    transition: 0.5s;
}

button:hover span{
    width: 100%;
}