:root {
    --primary-color: #2563EB;
    --primary-hover: #1D4ED8;
    --primary-light: #EFF6FF;
    --secondary-color: #64748B;
    --secondary-hover: #475569;
    --secondary-light: #F1F5F9;
    --danger-color: #EF4444;
    --danger-hover: #DC2626;
    --success-color: #10B981;
    --success-light: #D1FAE5;
    --background-color: #F8FAFC;
    --card-bg: #FFFFFF;
    --text-main: #1E293B;
    --text-muted: #64748B;
    --border-color: #E2E8F0;
    --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
    --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
    --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 18px;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Inter', sans-serif;
}

body {
    background-color: var(--background-color);
    color: var(--text-main);
    display: flex;
    flex-direction: column;
    min-height: 100vh;
}

/* ========== Header ========== */
.header {
    background-color: var(--card-bg);
    padding: 1.25rem 2rem;
    display: flex;
    justify-content: space-between;
    align-items: center;
    box-shadow: var(--shadow-sm);
    position: sticky;
    top: 0;
    z-index: 100;
}

.logo {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--primary-color);
}

.nav ul {
    list-style: none;
    display: flex;
    gap: 2.5rem;
}

.nav a {
    text-decoration: none;
    color: var(--text-muted);
    font-weight: 500;
    transition: color 0.2s ease;
}

.nav a:hover,
.nav a.active {
    color: var(--primary-color);
}

.header-contact {
    display: flex;
    gap: 1.5rem;
}

.contact-link {
    text-decoration: none;
    color: var(--text-muted);
    display: flex;
    align-items: center;
    gap: 0.5rem;
    font-size: 0.9rem;
    font-weight: 500;
    transition: color 0.2s ease;
}

.contact-link:hover {
    color: var(--primary-color);
}

/* ========== Main ========== */
.main-content {
    flex: 1;
    display: flex;
    justify-content: center;
    align-items: flex-start;
    padding: 3rem 1rem;
}

.converter-container {
    max-width: 650px;
    width: 100%;
    display: flex;
    flex-direction: column;
    gap: 2rem;
}

/* ========== Category Tabs ========== */
.category-tabs {
    display: flex;
    gap: 0.5rem;
    background-color: var(--card-bg);
    padding: 0.5rem;
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-md);
    overflow-x: auto;
}

.tab {
    flex: 1;
    padding: 0.875rem 1rem;
    border: none;
    background-color: transparent;
    border-radius: var(--radius-md);
    font-weight: 600;
    font-size: 0.9rem;
    color: var(--text-muted);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 0.5rem;
    white-space: nowrap;
    transition: all 0.25s ease;
}

.tab:hover {
    background-color: var(--secondary-light);
    color: var(--text-main);
}

.tab.active {
    background-color: var(--primary-color);
    color: white;
    box-shadow: 0 4px 12px rgba(37, 99, 235, 0.25);
}

/* ========== Card ========== */
.card {
    background-color: var(--card-bg);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-lg);
    padding: 2.5rem;
    transition: box-shadow 0.3s ease;
}

.card:hover {
    box-shadow: 0 20px 25px -5px rgba(0, 0, 0, 0.1), 0 10px 10px -5px rgba(0, 0, 0, 0.04);
}

.card-header {
    margin-bottom: 1.5rem;
}

.card-header h3 {
    font-size: 1.1rem;
    color: var(--text-main);
    font-weight: 700;
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

.flex-between {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

/* ========== Converter Body ========== */
.converter-body {
    display: flex;
    flex-direction: column;
    gap: 1.25rem;
}

.field-group {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
}

.field-label {
    font-weight: 600;
    font-size: 0.85rem;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.input-row {
    display: flex;
    gap: 1rem;
}

.input-wrapper {
    flex: 1;
    border: 2px solid var(--border-color);
    border-radius: var(--radius-md);
    background-color: var(--background-color);
    display: flex;
    align-items: center;
    padding: 0 1rem;
    transition: all 0.3s ease;
}

.input-wrapper:focus-within {
    border-color: var(--primary-color);
    background-color: white;
    box-shadow: 0 0 0 4px rgba(37, 99, 235, 0.1);
}

.result-wrapper {
    border-color: var(--success-color);
    background-color: var(--success-light);
}

.result-wrapper:focus-within {
    border-color: var(--success-color);
    box-shadow: 0 0 0 4px rgba(16, 185, 129, 0.1);
}

.form-input {
    flex: 1;
    border: none;
    background: transparent;
    padding: 1rem 0;
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--text-main);
    outline: none;
    width: 100%;
    /* Hide arrows */
    -moz-appearance: textfield;
}

.form-input::-webkit-outer-spin-button,
.form-input::-webkit-inner-spin-button {
    -webkit-appearance: none;
    margin: 0;
}

.form-input::placeholder {
    color: #94A3B8;
    font-weight: 400;
}

.form-select {
    width: 160px;
    padding: 1rem 1.25rem;
    border: 2px solid var(--border-color);
    border-radius: var(--radius-md);
    background-color: var(--background-color);
    font-size: 1rem;
    font-weight: 600;
    color: var(--text-main);
    cursor: pointer;
    outline: none;
    transition: all 0.3s ease;
    appearance: none;
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='12' fill='%2364748B' viewBox='0 0 16 16'%3E%3Cpath d='M8 11L3 6h10l-5 5z'/%3E%3C/svg%3E");
    background-repeat: no-repeat;
    background-position: right 1rem center;
}

.form-select:focus {
    border-color: var(--primary-color);
    box-shadow: 0 0 0 4px rgba(37, 99, 235, 0.1);
}

/* Swap Button */
.swap-row {
    display: flex;
    justify-content: center;
    padding: 0.25rem 0;
}

.btn-swap {
    width: 48px;
    height: 48px;
    border-radius: 50%;
    border: 2px solid var(--border-color);
    background-color: white;
    color: var(--primary-color);
    font-size: 1.1rem;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all 0.3s ease;
}

.btn-swap:hover {
    background-color: var(--primary-color);
    color: white;
    border-color: var(--primary-color);
    transform: rotate(180deg);
}

.btn-swap:active {
    transform: rotate(180deg) scale(0.9);
}

/* Validation */
.validation-message {
    color: var(--danger-color);
    font-size: 0.85rem;
    font-weight: 500;
    min-height: 20px;
    margin-top: 0.25rem;
}

/* Action Buttons */
.action-buttons {
    display: flex;
    gap: 0.75rem;
    margin-top: 0.75rem;
    flex-wrap: wrap;
}

.btn {
    padding: 0.875rem 1.5rem;
    border: none;
    border-radius: var(--radius-sm);
    font-size: 0.95rem;
    font-weight: 600;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 0.5rem;
    transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
}

.btn:active {
    transform: scale(0.96);
}

.btn-primary {
    background-color: var(--primary-color);
    color: white;
    flex: 1;
}

.btn-primary:hover {
    background-color: var(--primary-hover);
    box-shadow: 0 4px 12px rgba(37, 99, 235, 0.3);
}

.btn-secondary {
    background-color: var(--secondary-light);
    color: var(--text-main);
}

.btn-secondary:hover {
    background-color: #E2E8F0;
}

.btn-danger-outline {
    background-color: transparent;
    color: var(--text-muted);
    border: 1px solid var(--border-color);
}

.btn-danger-outline:hover {
    background-color: #FEF2F2;
    border-color: var(--danger-color);
    color: var(--danger-color);
}

.btn-icon {
    background-color: transparent;
    border: none;
    color: var(--text-muted);
    cursor: pointer;
    font-size: 0.95rem;
    padding: 0.5rem;
    border-radius: var(--radius-sm);
    transition: all 0.2s ease;
}

.btn-icon:hover {
    color: var(--danger-color);
    background-color: #FEF2F2;
}

/* Formula Box */
.formula-box {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    padding: 1rem 1.25rem;
    background-color: var(--primary-light);
    border-radius: var(--radius-sm);
    color: var(--primary-color);
    font-size: 0.95rem;
    font-weight: 500;
    margin-top: 1.5rem;
    animation: fadeIn 0.3s ease forwards;
}

.formula-box.hidden {
    display: none;
}

/* ========== History ========== */
.history-list {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
}

.history-item {
    padding: 0.875rem 1rem;
    background-color: var(--background-color);
    border-radius: var(--radius-sm);
    border: 1px solid var(--border-color);
    font-size: 0.9rem;
    font-weight: 500;
    color: var(--text-main);
    display: flex;
    align-items: center;
    gap: 0.75rem;
    animation: fadeIn 0.3s ease forwards;
}

.history-item i {
    color: var(--primary-color);
    font-size: 0.8rem;
}

.empty-state {
    text-align: center;
    padding: 2rem 1rem;
    color: var(--text-muted);
}

.empty-state.hidden {
    display: none;
}

/* ========== Toast ========== */
.toast {
    position: fixed;
    bottom: 2rem;
    right: 2rem;
    background-color: var(--text-main);
    color: white;
    padding: 1rem 1.5rem;
    border-radius: var(--radius-sm);
    display: flex;
    align-items: center;
    gap: 0.75rem;
    box-shadow: var(--shadow-lg);
    z-index: 1000;
    transition: all 0.4s cubic-bezier(0.68, -0.55, 0.265, 1.55);
    transform: translateY(150%);
    opacity: 0;
}

.toast.show {
    transform: translateY(0);
    opacity: 1;
}

.toast.success i {
    color: var(--success-color);
}

.toast.danger i {
    color: var(--danger-color);
}

/* ========== Footer ========== */
.footer {
    background-color: var(--card-bg);
    padding: 2rem;
    text-align: center;
    border-top: 1px solid var(--border-color);
    margin-top: auto;
}

.text-danger {
    color: var(--danger-color);
}

.footer-contact {
    display: flex;
    justify-content: center;
    gap: 2rem;
    font-size: 0.95rem;
    margin-top: 1rem;
}

.footer-contact a {
    color: var(--text-muted);
    text-decoration: none;
    transition: color 0.2s ease;
}

.footer-contact a:hover {
    color: var(--primary-color);
}

/* ========== Animations ========== */
@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(10px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}