main.css

* {
  box-sizing: border-box;
  padding: 0;
  margin: 0;}

body {
  font-family: 'Hi Melody', cursive;
  font-size: 150%}

a {
  text-decoration: none;
  display: inline-block}

.container {
  max-width: 28.125rem;
  margin: 0 auto;
  background: #dd0;
  padding: 2rem 5rem .6rem;}

h1 {
  padding: 0.4rem;
  background: #333;
  color: #fff;
  line-height: 1;
  text-align: center;
  font-weight: 500;}

.input-control {
  margin-bottom: 1.25rem;
  user-select: none;
  display: flex;
  align-items: center;
  position: relative;
  white-space: nowrap;}

input[type=checkbox] {
  visibility: hidden;
  margin-right: 5px;
  position:relative;}

input[type=checkbox]::before {
  content: "";
  display: inline-block;
  width: 14px;
  height: 14px;
  margin-top: -1.3px;
  background: #fff;
  border: 1px solid #ccc;
  box-sizing: border-box;
  visibility: visible;}

.fab, .far {
  vertical-align: middle;
  width: 1rem;
  font-size: 1rem;}

.check-icon {
  position: absolute;
  left: 1.1%;
  font-size: 7.8px;  
  z-index: 100;
  color: #fff;
  display: none;}

label {
  position:relative;
  margin-left: 4px;
  color: #333;}

label::before {
  content: "";
  width: 0;
  height: 14px;
  background: rgb(80, 140, 0);
  opacity: 0;
  position: absolute;
  top: 23%;
  transform: skewX(-10deg);
  transition: width .25s ease-in-out, opacity .5s ease-in-out;}

/* Button */
.btn-wrap {text-align: center}
#button {
  padding: .6rem 1rem;
  text-transform: uppercase;
  font-family: 'Hi Melody', cursive;
  font-size: 1rem;
  background: #fff;
  color: #333;
  border: 1px solid #ccc;
  outline: 0;
  transition: all .25s}
#button:active {border-color: #755;}

input[type=checkbox]:active::before {
  background: rgba(80, 140, 0, 0.4);}

/* Footer icons */
.social-l-box {
  text-align: center;
  white-space: nowrap;
  font-size: .87rem;
  margin-top: 2rem;} 
.social-l-box label {
  color: #755; 
  margin: 0 .4rem;
  cursor: pointer;}
.social-l-box .fab {
  font-size: inherit;
  width:auto;}

/* CHECKED FUNCs */
input[type=checkbox]:checked::before {
  background: rgb(80, 140, 0);
  border-color: rgba(150, 180, 0);}

input[type=checkbox]:checked ~ label::before {
  width: 100%;
  opacity: 0.4;
  transition: width .35s ease-in-out, opacity .25s ease-in-out;}

input[type=checkbox]:checked + .check-icon {display: inline;}