.header {
    background: linear-gradient(135deg, #0052cc, #33b5ff);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
    top: 0;
    padding: 5px 15px;
    min-width:195px;
    max-width:195px;
    height:100vh;
}

.header-content {
    text-align:center;
}

.logo-img {
    height: 70px;
    margin-top:20px;
}
.nav{
    text-align: left;
    margin: 20px 0;
}
.nav-list {
    list-style: none;
    margin: 0;
    padding: 0;
    gap:10px;
}

.nav-item {
    position: relative;
}

.nav-link {
    color: #fff;
    text-decoration: none;
    font-size: 1em;
    transition: background 0.3s ease, color 0.3s ease;
    border-radius: 5px;
    padding: 10px;
    font-weight:600;
}

.nav-link:hover {
    background: rgba(255, 255, 255, 0.2);
    color: #fff;
}

.dropdown-toggle::after {
    content: '►';
    font-size: 0.7em;
    margin-left: 5px;
}

.dropdown-menu {
    display: none;
    position: absolute;
    left: 100%;
    top: 20px;
    background: #fff;
    box-shadow: 0 4px 10px rgba(0, 0, 0, 0.1);
    list-style: none;
    padding: 0;
    border-radius: 5px;
    z-index: 1000;
    transition: all 0.3s ease;
}

.dropdown-item {
    display: block;
    color: #1a3c6e;
    text-decoration: none;
    padding: 10px 15px;
    font-size: 1em;
    transition: background 0.3s ease;
}

.dropdown-item:hover {
    background: #e6f3ff;
}

.submenu-title{
    font-weight: 600;
    padding-left: 12px;
    font-size: 1em;
    color: #1a3c6e;
}

.submenu {
    list-style: none;
    padding: 0;
    display: grid;
    grid-template-columns: repeat(2, 1fr); /* 4 cột đều nhau */
}

.submenu.active {
    display: block;
}

.menu-toggle {
    display: none;
    background: none;
    border: none;
    cursor: pointer;
    padding: 10px;
    grid-template-columns: repeat(2, 1fr);
}

.menu-icon,
.menu-icon::before,
.menu-icon::after {
    display: block;
    width: 25px;
    height: 3px;
    background: #fff;
    transition: all 0.3s ease;
}

.menu-icon::before,
.menu-icon::after {
    content: '';
    position: absolute;
}

.menu-icon::before {
    transform: translateY(-8px);
}

.menu-icon::after {
    transform: translateY(8px);
}

.menu-toggle.active .menu-icon {
    background: transparent;
}

.menu-toggle.active .menu-icon::before {
    transform: rotate(45deg);
}

.menu-toggle.active .menu-icon::after {
    transform: rotate(-45deg);
}

@media (min-width: 769px) {
    .dropdown:hover .dropdown-menu {
        display: grid;
        grid-template-columns: repeat(2, 1fr);
        min-width: max-content;
    }
    .nav-item{
        padding: 20px 0;
    }
}
/* Responsive Navigation */
@media (max-width: 768px) {
    
    .header{
        min-width:unset;
        max-width:unset;
        border-radius:0;
        height: unset;
    }
    
    .logo-img {
        height: 40px;
        margin-top: 0;
    }
    .nav-list {
        display: none;
        flex-direction: column;
        position: absolute;
        top: 53px;
        left: 0;
        width: 100%;
        background: linear-gradient(135deg, #007bff, #00c4ff);
        padding: 10px 0;
    }
    
    .header-content {
        margin: 0 auto;
        display: flex;
        align-items: center;
        justify-content: center;
    }

    .nav-list.active {
        display: flex;
    }

    .nav-item {
        width: 100%;
    }

    .nav-link {
        display: block;
        padding: 15px 20px;
        text-align: left;
    }
     .submenu {
        list-style: none;
        padding: 0;
        display: grid;
        grid-template-columns: repeat(2, 1fr); /* 4 cột đều nhau */
    }
    
    .submenu-title{
        font-size:0.9em;
    }
    
    .menu-toggle{
        display:block;
        position: absolute;
        right: 20px;
        transform: translateY(-50%);
    }
   .dropdown-menu {
        display: none;
        position: static;
        background: rgba(255, 255, 255, 0.95);
        box-shadow: none;
        margin: 2px 10px;
        border-radius: 5;
        width: width: 89%;
    }

    .dropdown-menu.active {
        display: grid;
        grid-template-columns: repeat(2, 1fr);
    }
}

@media (max-width: 480px) {
    .logo-img {
        height: 40px;
        margin-top: 0;
    }

    .nav-link {
        font-size: 1em;
        padding: 10px 10px;
    }

    .dropdown-item {
        font-size: 0.9em;
        padding: 5px 5px;
    }
}