*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body{
    background-color: #f8f9fa;
    color: #333;
    line-height: 1.6;
}

header{
    background-color: #e7f5ff;
    padding: 1rem 2rem;
    box-shadow: 0 2px 10px rgba(0,0,0,0.1);
    position: sticky;
    top: 0;
    z-index: 100;
}
.header-container{
    display: flex;
    justify-content: space-between;
    align-items: center;
    max-width: 1200px;
    margin: 0 auto;
}
.logo{
    font-size: 1.8rem;
    font-weight: bold;
    color: #228be6;
    text-decoration: none;
    display: flex;
    align-items: center;
}

nav ul{
    display: flex;
    list-style: none;
}
nav ul li{
    margin-left: 1.5rem;
}
nav ul li a{
    text-decoration: none;
    color: #495057;
    font-weight: 500;
    opacity: 0.8;
}
nav ul li a:hover{
    opacity: 1;
}

.auth-buttons{
    display: flex;
    gap: 1rem;
}
.btn{
    padding: 0.5rem 1.2rem;
    border-radius: 5px;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.3s;
    border: none;
    font-size: 0.95rem;
    
}
.btn-login{
    background-color: #228be6;
    color: white;
    opacity: 1;
}
.btn-login:hover{
    opacity: 0.8;
}

.btn-register{
    background-color: #228be6;
    color: white;
    opacity: 1;
}
.btn-register:hover{
    opacity: 0.8;
}

/* header */

main{
    max-width: 1200px;
    margin: 2rem auto;
    padding: 0 2rem;
}
.hero{
    background-color: #d0ebff;
    padding: 4rem 2rem;
    border-radius: 10px;
    text-align: center;
    margin-bottom: 3rem;
}
.hero h1{
    font-size: 2.5rem;
    color: #1864ab;
    margin-bottom: 1rem;
}
.hero p{
    font-size: 1.1rem;
    color: #495057;
    max-width: 700px;
    margin: 0 auto 2rem;
}
.icon {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
    gap: 1.5rem;
    margin: 3rem auto;
    padding: 0 1rem;
}

.card {
    background-color: #ffffff;
    border-radius: 12px;
    padding: 2rem 1.5rem;
    box-shadow: 0 8px 24px rgba(0, 0, 0, 0.08);
    text-align: center;
    transition: transform 0.3s ease, box-shadow 0.3s ease;
}

.card:hover {
    transform: translateY(-6px);
    box-shadow: 0 12px 30px rgba(0, 0, 0, 0.12);
}

.card i {
    font-size: 2.5rem;
    color: #1864ab;
    margin-bottom: 1rem;
}

.card p {
    font-size: 1.2rem;
    font-weight: 600;
    color: #343a40;
}

.features{
    display: grid;
    grid-template-columns: repeat(auto-fit,minmax(100px,1fr));
    gap: 2rem;
    margin-bottom: 3rem;
}

.feature-card{
    background-color: white;
    border-radius: 8px;
    padding: 1.5rem;
    box-shadow: 0 10px 20px rgba(0,0,0,0.1);
    
}


.feature-card h3{
    color: #228be6;
    margin-bottom: 1rem;
}
.btn-handle{
    margin-top: 1rem;
}
/* main */
footer{
    background-color: #e7f5ff;
    padding: 3rem 2rem;
    margin-top: 3rem;
}

.footer-container{
    max-width: 1200px;
    margin: 0 auto;
    display: grid;
    grid-template-columns: repeat(auto-fit,minmax(250px,1fr));
    gap: 2rem;
}
.footer-logo{
    font-size: 1.5rem;
    font-weight: bold;
    color: #228be6;
    margin-bottom: 1rem;
    display: inline-block;
}

.footer-about p{
    margin-bottom: 1.5rem;
}
.footer-contact h3{
    color: #1864ab;
    margin-bottom: 1.5rem;
    font-size: 1.2rem;
}
.footer-links{
    margin-top: 2rem;
    margin-left: 3rem;
}
.footer-links ul{
    list-style: none;
    display: flex;
    flex-direction: column;

}
.footer-links ul li{
    margin-bottom: 0.8rem;
}
.footer-links ul li a{
    text-decoration: none;
    color: #495057;
    transition: color 0.3s;

}
.footer-links ul li a:hover{
    color: #228be6;
}
.copyright{
    text-align: center;
    padding-top: 2rem;
    margin-top: 2rem;
    border-top: 1px solid #d0ebff;
    color: #495057;
    font-size: 0.9rem;
}
@media (max-width: 480px) {
  .header-container {
    flex-direction: column;
    align-items: flex-start;
  }

  nav ul {
    flex-direction: column;
    gap: 10px;
  }

  .auth-buttons {
    flex-direction: column;
    width: 100%;
  }

  button, .btn {
    width: 100%;
  }

  .form-control {
    font-size: 1rem;
  }

  .container {
    padding: 10px;
  }
}

/* Mobile lớn / tablet nhỏ (≤ 768px) */
@media (max-width: 768px) {
  main {
    padding: 10px;
  }

  .dashboard-container,
  .teacher-container,
  .create-exercise-container {
    padding: 16px;
  }

  .form-control, .btn {
    font-size: 1rem;
  }
}

/* Tablet đến Laptop (≤ 992px) */
@media (max-width: 992px) {
  .dashboard-container {
    grid-template-columns: 1fr;
  }

  nav ul {
    gap: 16px;
    flex-wrap: wrap;
  }
}

/* Desktop lớn (> 1200px) */
@media (min-width: 1200px) {
  .container {
    max-width: 1200px;
  }
}