style.css

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

body {
  background: #dfe6e9;
  color: #333333;
  font-family: "Sen", sans-serif;
}

#app {
  max-width: 450px;
  width: 90%;
  background: #fff;
  margin: 5% auto;
  border-radius: 0.5rem;
  display: flex;
  flex-direction: column;
  padding: 1.5rem;
}

#todo-title {
  font-size: 1.5rem;
  margin-bottom: 0.2rem;
}

#todo-subtitle {
  font-size: 0.85rem;
  position: relative;
  width: 100%;
  padding-bottom: 1.4rem;
  margin-bottom: 1.4rem;
  color: #7a7a7a;
}

#todo-subtitle::after {
  content: "";
  width: 2.6rem;
  height: 5px;
  position: absolute;
  bottom: 0;
  left: 0;
  background: #2cafc9;
}

#todo-input {
  outline: none;
  border: 1px solid #eaeaea;
  height: 2.2rem;
  padding: 0 0.7rem;
  color: #333333;
  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.12), 0 1px 2px rgba(0, 0, 0, 0.2);
  transition: all 0.3s cubic-bezier(0.25, 0.8, 0.25, 1);
  margin-bottom: 1rem;
  border-radius: 2px;
}

#todo-input:focus,
#todo-input:hover {
  border: 1px solid #2cafc9;
}

#todo-input::placeholder {
  color: #bfbfbf;
}

#todo-submit {
  border: none;
  background: #2cafc9;
  color: #fff;
  width: 100%;
  height: 2.2rem;
  border-radius: 2px;
  outline: none;
  cursor: pointer;
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.2px;
  transition: all 0.3s cubic-bezier(0.25, 0.8, 0.25, 1);
  margin-bottom: 2rem;
}

#todo-submit:hover {
  background: #2098b0;
}

#todo-output {
  list-style-type: none;
  display: flex;
  flex-direction: column;
  row-gap: 0.6rem;
}

#todo-output li {
  font-size: 0.8rem;
  padding: 0.7rem 1rem;
  border-top-right-radius: 2px;
  border-bottom-right-radius: 2px;
  position: relative;
  border-left: 3px solid #dfe6e9;
  transition: all 0.3s cubic-bezier(0.25, 0.8, 0.25, 1);
  cursor: pointer;
}

#todo-output li::after {
  content: "\203A";
  font-size: 1.2rem;
  position: absolute;
  top: 50%;
  right: 0.7rem;
  color: #babebf;
  transform: translateY(-50%);
}

#todo-output li:hover {
  border-left: 3px solid #2098b0;
}

#todo-output li:hover::after {
  color: #2098b0;
}