/* Reset và cơ bản */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    line-height: 1.6;
    color: #333;
    background-color: #f4f4f4;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 20px;
}

header {
    text-align: center;
    margin-bottom: 30px;
    padding: 20px;
    background-color: #fff;
    border-radius: 8px;
    box-shadow: 0 2px 5px rgba(0,0,0,0.1);
}

header h1 {
    color: #2c3e50;
    margin-bottom: 10px;
}

header p {
    color: #7f8c8d;
}

.columns-container {
    display: flex;
    flex-wrap: wrap;
    gap: 20px;
    margin-bottom: 30px;
}

.column {
    flex: 1;
    min-width: 300px;
    background-color: #fff;
    padding: 20px;
    border-radius: 8px;
    box-shadow: 0 2px 5px rgba(0,0,0,0.1);
}

.column h2 {
    color: #3498db;
    margin-bottom: 20px;
    padding-bottom: 10px;
    border-bottom: 2px solid #ecf0f1;
}

.button-group {
    display: flex;
    flex-direction: column;
    gap: 15px;
}

.btn {
    display: inline-block;
    padding: 12px 20px;
    background-color: #3498db;
    color: white;
    text-decoration: none;
    border-radius: 5px;
    transition: all 0.3s ease;
    text-align: center;
    font-weight: 500;
    border: none;
    cursor: pointer;
    box-shadow: 0 2px 4px rgba(0,0,0,0.1);
}

.btn:hover {
    background-color: #2980b9;
    transform: translateY(-2px);
    box-shadow: 0 4px 8px rgba(0,0,0,0.15);
}

.btn:active {
    transform: translateY(0);
}

footer {
    text-align: center;
    padding: 20px;
    background-color: #fff;
    border-radius: 8px;
    box-shadow: 0 2px 5px rgba(0,0,0,0.1);
    margin-top: 20px;
}

/* Responsive design */
@media screen and (max-width: 768px) {
    .columns-container {
        flex-direction: column;
    }
    
    .column {
        min-width: 100%;
    }
    
    .container {
        padding: 10px;
    }
    
    header h1 {
        font-size: 1.5em;
    }
}