popup.css

body {
  font-family: 'Lato', 'Lucida Grande', 'Lucida Sans Unicode', Tahoma, Sans-Serif;
  width: 200px;
}

html:not([dir="rtl"]) header h1 {
  padding-left: 50px;
  padding-top: 14px;
  padding-bottom: 14px;
  background-position: left center;
}

header h1 {
  color: black;
  background-image: url(icon48.png);
  background-repeat: no-repeat;
  font-size: 1.5em;
  flex-grow: 1;
  padding: 0px;
  margin-bottom: 0px;
}

label {
  font-size: 1em;
}

input[type=checkbox] + label {
  display: block;
  margin: 0.2em;
  cursor: pointer;
  padding: 0.2em;
}

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

input[type=checkbox] + label:before {
  content: "\2714";
  border: 0.1em solid #000;
  border-radius: 0.2em;
  display: inline-block;
  width: 1em;
  height: 1em;
  padding-left: 0.2em;
  padding-bottom: 0.3em;
  margin-right: 0.4em;
  vertical-align: bottom;
  color: transparent;
  transition: .2s;
}

input[type=checkbox] + label:active:before {
  transform: scale(0);
}

input[type=checkbox]:checked + label:before {
  background-color: MediumSeaGreen;
  border-color: MediumSeaGreen;
  color: #fff;
}

input[type=checkbox]:disabled + label:before {
  transform: scale(1);
  border-color: #aaa;
}

input[type=checkbox]:checked:disabled + label:before {
  transform: scale(1);
  background-color: #bfb;
  border-color: #bfb;
}