style.css

*{
  box-sizing: border-box;
}

body{
  padding-left: 24px;
  padding-right: 24px;
  padding-top: 12px;
  padding-bottom: 12px;
  font-family: sans-serif;
}
.container{
  width: 30%;
}

ul{
  list-style: none;
  padding: 0;
  margin-top: 12px;
}

a{
  text-decoration: none;
  color: inherit;
}

h1.page-title{
  color: #2d5ca8;
}
small.page-subtitle{
  opacity: 0.2;
  padding: 24px;
  display: block;
  text-align: center;
}

ul li input[type=checkbox]{
  margin-right: 12px;
}

span#listStatus{
  display: block;
  padding-top: 12px;
}

input[type=text]{
  padding: 12px;
  border: 2px solid rgba(0,0,0,0.3);
  width: 100%;
  font-size: 14px;
  display: block;
  border-radius: 0px;
  -webkit-appearance: none;
}
input[type=text]:focus{
  outline: none;
}
#userInput{
  font-size: 20px;
  margin-right: 12px;
  width: 80%;
}
#userInputContainer button{
  font-size: 16px;
  background: #4286f4;
  border: 0px;
  color: #ffffff;
  cursor: pointer;
  padding-left: 20px;
  padding-right: 20px;
}
#userInputContainer button:hover{
  background: #2d5ca8;
}

#userInputContainer{
  display: flex;
  justify-content: space-between;
  margin-bottom: 24px;
}

.todo-section, .complete-section{
  padding: 12px;
  padding-left: 24px;
  border: 2px solid rgba(0,0,0,0.4);
  width: auto;
  display: block;
  margin-bottom: 12px;
}

.complete-section{
  border-color: rgba(0,0,0,0.2);
}

.complete-section h4{
  opacity: 0.2;
}
.complete-section ul li{
  opacity: 0.2;
  transition: 0.2s ease;
}

.complete-section ul > li:hover{
  opacity: 0.7;
  transition: 0.2s ease;
}

#alertBox{
  width: 100%;
  height: 100%;
  background-color: rgba(0,0,0,0.8);
  top: 0;
  left: 0;
  position: fixed;
  display: none;
}
#alertBox #alertBoxContent{
  width: 400px;
  top: 25%;
  position: relative;
  height: auto;
  padding: 24px;
  margin: 0 auto;
  background-color: #ffffff;
  box-shadow: 2px 2px 8px 0px rgba(0,0,0,0.4);
}
#alertBox #alertBoxMessage{
  display: block;
  font-size: 20px;
  padding-bottom: 24px;
}
#alertBox #alertBoxToDoName{
  font-weight: bold;
}
#alertBox button{
  width: 49%;
  display: inline-block;
  float: left;
  margin-top: 12px;
  appearance: none;
  -webkit-appearance: none;
  border: none;
  padding: 12px;
  cursor: pointer;
  font-size: 12px;
}
#alertBox button:focus{
  outline: 0;
}
#alertBox #alertBoxSubmit{
  background-color: #4286f4;
  color: #ffffff;
}
#alertBox #alertBoxSubmit:hover{
  background-color: #2d5ca8;
}
#alertBox #alertBoxClose{
  background-color: #ffffff;
  color: #4286f4;
  border: 1px solid #4286f4;
}
#alertBox #alertBoxButtonContainer{
  display: flex;
  justify-content: space-between;
  width: 100%;
}
#alertBoxStatus, #listStatus{
  padding-bottom: 12px;
  display: block;
}

@media screen and (max-width: 960px){
  .container{
    width: 100%;
  }
  #alertBox #alertBoxContent{
    width: 90%;
  }
}