:root {
    --bg: #f8fafc;
    --card-bg: #ffffff;
    --text: #0f172a;
    --muted: #64748b;
    --border: #e2e8f0;
    --primary: #3b82f6;
    --primary-light: #60a5fa;
    --secondary: #8b5cf6;
    --soft: #f1f5f9;
    --success: #10b981;
    --shadow: rgba(0, 0, 0, 0.08);
    --gradient-start: #6366f1;
    --gradient-end: #8b5cf6;
}

body.dark {
    --bg: #0f172a;
    --card-bg: #1e293b;
    --text: #f1f5f9;
    --muted: #94a3b8;
    --border: #334155;
    --primary: #60a5fa;
    --primary-light: #93c5fd;
    --secondary: #a78bfa;
    --soft: #1e293b;
    --success: #34d399;
    --shadow: rgba(0, 0, 0, 0.2);
    --gradient-start: #818cf8;
    --gradient-end: #c084fc;
}



* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: "Segoe UI", system-ui, sans-serif;
}

body {
    min-height: 100vh;
    background: var(--bg);
    display: flex;
    align-items: center;
    justify-content: center;
}

.app {
    width: 100%;
    padding: 16px;
}

/* Card */
.card {
    max-width: 380px;
    margin: auto;
    background: var(--card-bg);
    padding: 24px;
    border-radius: 16px;
    box-shadow: 0 20px 40px rgba(0, 0, 0, 0.08);
    border: 1px solid var(--border);
    position: relative;
    overflow: hidden;
}

.card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 4px;
    background: linear-gradient(90deg, var(--gradient-start), var(--gradient-end));
}

/* title */
.title {
    text-align: center;
    color: var(--text);
    margin-bottom: 24px;
    font-size: 28px;
    font-weight: 700;
    background: linear-gradient(90deg, var(--gradient-start), var(--gradient-end));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.field {
    margin-bottom: 20px;
}

.field label {
    display: block;
    margin-bottom: 8px;
    font-size: 14px;
    color: var(--muted);
}

/* input group */
.input-group {
    display: flex;
    align-items: center;
    border-radius: 12px;
    border: 2px solid var(--border);
    overflow: hidden;
    transition: all 0.3s ease;
}
.input-group:focus-within {
    border-color: var(--primary);
    box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.1);
}

.currency {
    padding: 14px 16px;
    background: linear-gradient(135deg, var(--soft), var(--card-bg));
    color: var(--primary);
    font-weight: 700;
    font-size: 18px;
}

.input-group input {
    flex: 1;
    padding: 14px;
    font-size: 18px;
    outline: none;
    border: none;
    background: transparent;
    color: var(--text);
    font-weight: 500;
}

.input:placeholder {
    color: var(--muted);
    opacity: 0.6;
}

input:-webkit-autofill,
input:-webkit-autofill:hover,
input:-webkit-autofill:focus {
    -webkit-box-shadow: 0 0 0px 1000px var(--card-bg) inset;
    -webkit-text-fill-color: var(--text);
}

/* tip buttons */
.tip-buttons {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 12px;
    margin-bottom: 12px;
}

.tip-buttons button {
    padding: 14px;
    border-radius: 12px;
    border: 2px solid var(--border);
    background: var(--soft);
    color: var(--text);
    font-weight: 600;
    cursor: pointer;
    transition: all 0.3s ease;
    position: relative;
    overflow: hidden;
}

.tip-buttons button:hover {
    transform: translateY(-2px);
    background-color: var(--primary);
    box-shadow: 0 4px 12px rgba(59, 130, 246, 0.2);
}

.tip-buttons button.active {
    background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
    color: white;
    border: none;
    box-shadow: 0 4px 15px rgba(99, 102, 241, 0.3);
}

/* custom tip */
.custom-tip {
    width: 100%;
    padding: 12px;
    border-radius: 10px;
    border: 1px solid var(--border);
    font-size: 14px;
    outline: none;
}

.custom-tip:focus {
    border-color: var(--primary);
    box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.3);
}

/* people control */
.people-control {
    display: flex;
    align-items: center;
    justify-content: space-between;
    border: 2px solid var(--border);
    border-radius: 12px;
    padding: 8px 16px;
    background: var(--soft);
    max-width: 200px;
    margin: 0 auto;
}

.people-control button {
    width: 40px;
    height: 40px;
    border-radius: 10px;
    border: none;
    background: var(--primary);
    color: color;
    font-size: 20px;
    font-weight: bold;
    cursor: pointer;
    transition: all 0.3s ease;
}

.people-control button:hover {
    background: var(--secondary);
    transform: scale(1.1);
}

.people-control button:active {
    transform: scale(0.95);
}

.people-control span {
    font-size: 20px;
    font-weight: 700;
    color: var(--text);
    min-width: 40px;
    text-align: center;
}

/* result section */
.result {
    background: linear-gradient(135deg, var(--soft), var(--card-bg));
    border-radius: 16px;
    padding: 24px;
    margin-bottom: 24px;
    border: 2px solid var(--border);
}

.result .row {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 16px;
    padding-bottom: 16px;
    border-bottom: 1px dashed var(--border);
}

.result .row:last-child {
    margin-bottom: 0;
    padding-bottom: 0;
    border-bottom: none;
}

.result span {
    color: var(--muted);
    font-size: 16px;
    font-weight: 500;
}

.result strong {
    color: var(--success);
    font-size: 24px;
    font-weight: 700;
    background: linear-gradient(90deg, var(--success), #22c55e);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

/* reset button */
.reset-btn {
    width: 100%;
    padding: 16px;
    border-radius: 12px;
    border: none;
    background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
    color: white;
    font-weight: 600;
    font-size: 16px;
    cursor: pointer;
    transition: all 0.3s ease;
    letter-spacing: 0.5px;
}

.reset-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 8px 20px rgba(99, 102, 241, 0.3);
}

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

.theme-toggle {
    background: var(--soft);
    border: 1px solid var(--border);
    font-size: 20px;
    cursor: pointer;
    float: right;
    margin-top: -65px;
    width: 40px;
    height: 40px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all 0.3s ease;
}

.theme-toggle:hover {
    transform: rotate(30deg);
    color: white;
}

body, .card, button, input, .result {
    transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease;
}