example.css

* {
  box-sizing: border-box;
}
body {
  margin: auto;
  font-family: Arial, sans-serif;
  background: #f3f5f8;
  padding: 32px;
}
todo-app {
  display: block;
  margin: auto;
  max-width: 500px;
}
todo-app .add-item input {
  padding: 0.5em 0.8em;
  margin: 12px 0;
  font-size: larger;
  width: 100%;
}
todo-app > footer {
  margin-top: 48px;
  font-size: smaller;
  text-align: center;
}
todo-item {
  display: flex;
  padding: 8px 0;
  border-bottom: 1px solid #eee;
  gap: 8px;
}
todo-item .title {
  flex: 1;
}
todo-item input[type="text"] {
  width: 100%;
}
card {
  background: white;
  border-radius: 8px;
  box-shadow: 0 2px 12px #1112;
  display: block;
  padding: 32px;
}
h1 {
  margin: 0;
}
button.flat {
  appearance: none;
  background: none;
  border: none;
}
button.selected {
  filter: invert(1);
}
todo-list > footer {
  justify-content: space-between;
  margin-top: 24px;
}
.flex {
  display: flex;
}
.muted {
  opacity: 0.5;
}