body {
  font-family: Arial;
  text-align: center;
  margin: 20px;
}

table {
  margin: 20px auto;
  border-collapse: collapse;
}

th, td {
  border: 2px solid #115ff4;
  padding: 15px 25px;
  min-width: 100px;
  cursor: pointer;
  text-align: center;
  font-size: 18px;
}

th {
  background-color: #115ff4;
  color: white;
}

td {
  background-color: #28a200;
  color: white;
  transition: background-color 0.3s;
}

td.viewed {
  background-color: #8d2ab5;
  text-decoration: line-through;
  color: white;
}

button {
  padding: 10px 20px;
  font-size: 16px;
  cursor: pointer;
}