styles.css

*{
  box-sizing: border-box;
}

html, body{
  font-family:'Helvetica Neue', Helvetica, sans-serif;
  height: 100%;
  width: 100%;
  margin: 0;
}

body{
  padding: 1em;
}

label{
  border: 1px solid #ccc;
  padding: .25em .5em;
  display: inline-block;
  border-radius: 3px;
  cursor: pointer;
}

h1{
  text-align: center;
  margin: 0;
  padding: 0 1em;
  font-size: 3em;
  letter-spacing: -.025em;
  position: absolute;
  left: 0;
  top: 50%;
  width: 100%;
  transform: translateY(-50%);
}

input[type="checkbox"]{
  display: none;
}

:checked + label{
  background: #000;
  color: #fff;
  border: 1px solid #000;
}

.inline-block{
  display: inline-block;
  margin: .25em;
}

.bottom-right{
  position: absolute;
  bottom: 1em;
  right: 1em;
  font-weight: bold;
}

.dark {
  background: #000;
  color: #fff;
}

.bulb{
  position: absolute;
  bottom: 1em;
  left: 1em;
  cursor: pointer;
}

:checked + label{
  color: #fff;
  background: #0073e6;
  border: 1px solid #0073e6;
}