:root {
    --bg-color: #f3f4f6;
    --app-bg: #ffffff;
    --text-color: #1f2937;
    --primary-color: #3b82f6;
    --secondary-color: #e5e7eb;
    --accent-color: #60a5fa;
    --key-bg: #f9fafb;
    --key-hover: #e5e7eb;
    --op-bg: #dbeafe;
    --op-text: #2563eb;
    --equals-bg: #3b82f6;
    --equals-text: #ffffff;
    --display-bg: #f3f4f6;
    --shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
}

[data-theme="dark"] {
    --bg-color: #111827;
    --app-bg: #1f2937;
    --text-color: #f9fafb;
    --primary-color: #60a5fa;
    --secondary-color: #374151;
    --accent-color: #3b82f6;
    --key-bg: #374151;
    --key-hover: #4b5563;
    --op-bg: #1e3a8a;
    --op-text: #93c5fd;
    --equals-bg: #2563eb;
    --equals-text: #ffffff;
    --display-bg: #111827;
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

body {
    font-family: 'Inter', sans-serif;
    background-color: var(--bg-color);
    color: var(--text-color);
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    transition: background-color 0.3s, color 0.3s;
}

.app-container {
    background-color: var(--app-bg);
    border-radius: 20px;
    box-shadow: var(--shadow);
    width: 100%;
    max-width: 400px; /* Default for basic/mobile */
    padding: 20px;
    transition: max-width 0.3s ease;
    overflow: hidden;
}

/* Header & Nav */
header {
    display: flex;
    flex-direction: column;
    gap: 15px;
    margin-bottom: 20px;
}

header h1 {
    font-size: 1.5rem;
    font-weight: 600;
    text-align: center;
}

header h2 {
    text-align: center;
    color: #60a5fa;
}

a{
    color:inherit;
    text-decoration: none ;
}

.mode-switch {
    display: flex;
    gap: 5px;
    background-color: var(--display-bg);
    padding: 5px;
    border-radius: 10px;
}

.tab-btn {
    background: none;
    border: none;
    padding: 8px 12px;
    border-radius: 8px;
    cursor: pointer;
    color: var(--text-color);
    font-size: 0.85rem;
    font-weight: 500;
    white-space: nowrap;
    transition: background-color 0.2s;
    flex: 1;
}

.tab-btn.active {
    background-color: var(--app-bg);
    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
    color: var(--primary-color);
}

#theme-toggle {
    position: absolute;
    top: 20px;
    right: 20px;
    background: none;
    border: none;
    cursor: pointer;
    color: var(--text-color);
    padding: 5px;
    border-radius: 50%;
}

#theme-toggle:hover {
    background-color: var(--secondary-color);
}

/* Display */
.display {
    background-color: var(--display-bg);
    border-radius: 15px;
    padding: 20px;
    margin-bottom: 20px;
    text-align: right;
    min-height: 100px;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
    word-break: break-all;
}

.previous-operand {
    color: var(--text-color);
    opacity: 0.7;
    font-size: 1rem;
    min-height: 1.2rem;
}

.current-operand {
    color: var(--text-color);
    font-size: 2.5rem;
    font-weight: 500;
}

/* Keypads */
.calculator-mode {
    display: none;
    animation: fadeIn 0.3s ease;
}

.calculator-mode.active {
    display: block;
}

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

.keypad {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 12px;
}

.advanced-keys {
    grid-template-columns: repeat(5, 1fr);
}

button {
    font-family: inherit;
}

.keypad button {
    height: 60px;
    border-radius: 12px;
    border: none;
    font-size: 1.25rem;
    font-weight: 500;
    cursor: pointer;
    background-color: var(--key-bg);
    color: var(--text-color);
    transition: filter 0.2s, transform 0.1s;
}

.keypad button:hover {
    filter: brightness(0.95);
}

.keypad button:active {
    transform: scale(0.95);
}

.btn-op {
    background-color: var(--op-bg) !important;
    color: var(--op-text) !important;
}

.btn-equals {
    background-color: var(--equals-bg) !important;
    color: var(--equals-text) !important;
}

.btn-clear, .btn-delete {
    color: #ef4444;
}

.span-2 {
    grid-column: span 2;
}

/* Converters */
.converter-container {
    padding: 10px;
}

.converter-group {
    margin-bottom: 20px;
}

.converter-group label {
    display: block;
    margin-bottom: 8px;
    font-size: 0.9rem;
    font-weight: 500;
}

select, input {
    width: 100%;
    padding: 12px;
    border-radius: 10px;
    border: 1px solid var(--secondary-color);
    background-color: var(--display-bg);
    color: var(--text-color);
    font-family: inherit;
    font-size: 1rem;
    outline: none;
}

select:focus, input:focus {
    border-color: var(--primary-color);
}

.converter-inputs {
    display: flex;
    flex-direction: column;
    gap: 15px;
}

.input-group {
    display: flex;
    flex-direction: column;
    gap: 5px;
}

.equals-sign {
    text-align: center;
    font-size: 1.5rem;
    color: var(--primary-color);
}

.exchange-icon {
    text-align: center;
    font-size: 1.5rem;
    transform: rotate(90deg);
    color: var(--primary-color);
    margin: 10px 0;
}

.result-display {
    margin-top: 20px;
    text-align: center;
    font-size: 1.5rem;
    font-weight: 600;
    color: var(--primary-color);
    padding: 20px;
    background-color: var(--display-bg);
    border-radius: 15px;
}

.last-updated {
    text-align: center;
    margin-top: 10px;
    font-size: 0.8rem;
    opacity: 0.7;
}

/* Responsive adjustments */
@media (min-width: 600px) {
    .app-container {
        max-width: 450px;
    }
    
    /* Expand for advanced mode */
    body:has(#advanced.active) .app-container {
        max-width: 600px;
    }
}