index.css

html, body {
  background: #FAFAFA;
  color: #212121;
  font-family: "游ゴシック", YuGothic, "ヒラギノ角ゴ ProN", "Hiragino Kaku Gothic ProN", sans-serif;
}

#app {
  max-width: 680px;
  margin: auto;
}

#app h1 {
  text-align: center;
}

#app .addtask {
  height: 2.0em;
  display: flex;
}

#app .addtask input {
  font-family: inherit;
  flex-grow: 1;
  max-width: 100%;
  border: #F0F0F0 1px solid;
  border-radius: 3px 0 0 3px;
  padding: 0 1.0em;
}

#app .addtask button {
  font-family: inherit;
  background: #388E3C;
  color: #FAFAFA;
  padding: 5px 10px;
  border: none;
  border-radius: 0 3px 3px 0;
}

#app .tasklist {
  list-style-type: none;
  border: #F0F0F0 1px solid;
  border-radius: 3px;
  padding: 0;
  margin: 0;
}

#app .task {
  padding: 1.0em 3.0em;
  line-height: 1.5em;
}

#app .task label {
  position: relative;
  display: inline-block;
  margin: auto;
}

#app .task label > input[type="checkbox"] {
  display: none;
}

#app .task label:before {
  content: "";
  margin-bottom: -0.25em;
  display: inline-block;
  width: 1.0em;
  height: 1.0em;
  background: #FFF;
  border: #757575 1px solid;
  border-radius: 3px;
}

#app .task:nth-child(2n + 1) {
  background: #F0F0F0;
}

#app .task:nth-child(2n) {
  background: #FAFAFA;
}

#app .task.is-done {
  opacity: 0.5;
}

#app .task.is-done label:after {
  content: "";
  position: absolute;
  top: 6px;
  left: 2px;
  display: inline-block;
  width: 10px;
	height: 5px;
  border-left: 3px solid #1B5E20;
	border-bottom: 2px solid #1B5E20;
  transform: rotate(-45deg);
}