style.css

body {
    margin: auto;
}

.my-container {
    display: flex;
    flex-direction: column;
    position: absolute;
    left: 35%;
    top: 30%;
    width: 30%;
    background-color: red;
}

.header {
    display: flex;

}

.btn-add {
    padding-left: 20px;
    padding-right: 20px;
    border-radius: 0px;
}



.task {
    display: flex;
    align-items: center;
    justify-content: right;
    height: 60px;
    padding-left: 20px;
    padding-right: 20px;
    background-color: wheat;
    border: 0.1px solid gray;
}

.chb {
    display: none;
}

label {
    position: relative;
    margin: 0px;
}

span.checkbox {
    width: 50px;
}
 
span::before, span::after {
  content: '';
  position: absolute;
  top: 0;
  bottom: 0;
  margin: auto;
}

span.checkbox::before {
    width: 30px;    
    height: 30px;
    background-color: white;
    box-sizing: border-box;
    border: 3px solid transparent;
    transition: border-color .2s;
}


span.checkbox:hover::before {
    border: 3px solid #F0FF76;
}


span.checkbox::after {
    content: "\2713";
    font-family: 'FontAwesome';
    font-size: 25px;
    top: -5px;
    color: transparent;
    transition: color .2s;
}

input[type="checkbox"]:checked + label span.checkbox::after {
    color: green;
}