how to design checkbox in CSS

custom-checkbox.css

body #gform_wrapper_FORMID .gfield_checkbox li input[type=checkbox] {
    opacity: 0;
    position: absolute;   
}
body #gform_wrapper_FORMID .gfield_checkbox li input[type=checkbox] + label{
    position: relative;
    overflow: hidden;
    padding-left: 35px;
    cursor: pointer;
    display: inline-block;
}

body #gform_wrapper_FORMID .gfield_checkbox li input[type=checkbox] + label:before,
body #gform_wrapper_FORMID .gfield_checkbox li input[type=checkbox] + label:after{
    content: '';
    position: absolute;
    left: 0;
    z-index: 1;
    -webkit-transition: .2s;
    transition: .2s;
}
/* after */
body #gform_wrapper_FORMID .gfield_checkbox li input[type=checkbox]:not(:checked) + label:after{
    width: 20px;
    height: 20px;
    border: 2px solid #000;
    top: 0px;
}
body #gform_wrapper_FORMID .gfield_checkbox li input[type=checkbox]:checked + label:after{
    top: 0;
    width: 20px;
    height: 20px;
    border: 2px solid #26a69a;
    background-color: #26a69a;
    z-index: 0;
	border-radius: 2px
}
/* before */
body #gform_wrapper_FORMID .gfield_checkbox li input[type=checkbox]:not(:checked) + label:before{
	    width: 0;
    height: 0;
    border: 3px solid transparent;
    left: 6px;
    top: 10px;
    -webkit-transform: rotateZ(37deg);
    transform: rotateZ(37deg);
    -webkit-transform-origin: 100% 100%;
    transform-origin: 100% 100%;
}
body #gform_wrapper_FORMID .gfield_checkbox li input[type=checkbox]:checked + label:before{
	    top: 0;
    left: 1px;
    width: 8px;
    height: 13px;
	margin-top:3px;
    border-top: 2px solid transparent;
    border-left: 2px solid transparent;
    border-right: 2px solid #fff;
    border-bottom: 2px solid #fff;
    -webkit-transform: rotateZ(37deg);
    transform: rotateZ(37deg);
    -webkit-transform-origin: 100% 100%;
    transform-origin: 100% 100%;
}