body {
  font-family: Arial, sans-serif;
  margin: 0;
  padding: 0;
  background: #f2f2f2;
}
.hidden { display: none; }
header {
  background: #2a3f54;
  color: #fff;
  padding: 1em;
  display: flex;
  justify-content: space-between;
  align-items: center;
}
aside {
  background: #34495e;
  width: 200px;
  float: left;
  height: 100vh;
  color: white;
}
aside ul {
  list-style: none;
  padding: 0;
}
aside li {
  padding: 1em;
}
aside li a {
  color: white;
  text-decoration: none;
}
main {
  margin-left: 200px;
  padding: 2em;
  background: #fff;
  min-height: 100vh;
}
table {
  border-collapse: collapse;
  width: 100%;
}
th, td {
  border: 1px solid #ccc;
  padding: 0.5em;
}
button {
  padding: 0.5em 1em;
  margin-top: 1em;
  cursor: pointer;
}