:root {
    --light-bg: #87CEEB; /* Sky Blue */
    --light-road: #6b6b6b;
    --light-lines: #ffffff;
    --light-text: #000000;
    --light-container-bg: rgba(255, 255, 255, 0.9);
    --light-input-bg: #ffffff;
    --light-button-bg: #4CAF50;
    --light-button-text: #ffffff;
    --light-result-bg: #f0f0f0;
    --dark-bg: #0c0c1c; /* Night Sky */
    --dark-road: #333333;
    --dark-lines: #a9a9a9;
    --dark-text: #ffffff;
    --dark-container-bg: rgba(0, 0, 0, 0.8);
    --dark-input-bg: #444444;
    --dark-button-bg: #FFA500; /* Orange */
    --dark-button-text: #000000;
    --dark-result-bg: #222222;
}

[data-theme="light"] {
    --bg-color: var(--light-bg);
    --road-color: var(--light-road);
    --lines-color: var(--light-lines);
    --text-color: var(--light-text);
    --container-bg-color: var(--light-container-bg);
    --input-bg-color: var(--light-input-bg);
    --button-bg-color: var(--light-button-bg);
    --button-text-color: var(--light-button-text);
    --result-bg-color: var(--light-result-bg);
}

[data-theme="dark"] {
    --bg-color: var(--dark-bg);
    --road-color: var(--dark-road);
    --lines-color: var(--dark-lines);
    --text-color: var(--dark-text);
    --container-bg-color: var(--dark-container-bg);
    --input-bg-color: var(--dark-input-bg);
    --button-bg-color: var(--dark-button-bg);
    --button-text-color: var(--dark-button-text);
    --result-bg-color: var(--dark-result-bg);
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
    margin: 0;
    padding: 0;
    background-color: var(--bg-color);
    color: var(--text-color);
    transition: background-color 0.5s, color 0.5s;
    overflow-x: hidden;
    position: relative;
}

.highway {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: -1;
}

.road {
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 60%;
    background-color: var(--road-color);
    perspective: 100px;
}

.lines {
    position: absolute;
    width: 100%;
    height: 100%;
    overflow: hidden;
}

.line {
    position: absolute;
    left: 50%;
    transform: translateX(-50%);
    width: 5px;
    height: 30px;
    background-color: var(--lines-color);
    animation: moveLines 2s linear infinite;
}

.line:nth-child(2) { animation-delay: 0.5s; }
.line:nth-child(3) { animation-delay: 1s; }
.line:nth-child(4) { animation-delay: 1.5s; }

@keyframes moveLines {
    from { bottom: -30px; }
    to { bottom: 100%; }
}

.container {
    max-width: 500px;
    margin: 50px auto;
    padding: 30px;
    background-color: var(--container-bg-color);
    border-radius: 15px;
    box-shadow: 0 0 20px rgba(0,0,0,0.2);
    backdrop-filter: blur(10px);
}

h1 {
    text-align: center;
    margin-bottom: 30px;
}

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

.input-group.checkbox {
    display: flex;
    align-items: center;
}

.input-group.checkbox input {
    width: auto;
    margin-right: 10px;
}

label {
    display: block;
    margin-bottom: 5px;
}

input, select {
    width: 100%;
    padding: 10px;
    border: 1px solid #ccc;
    border-radius: 5px;
    background-color: var(--input-bg-color);
    color: var(--text-color);
    box-sizing: border-box;
}

select {
    margin-bottom: 10px;
}

button {
    width: 100%;
    padding: 15px;
    border: none;
    border-radius: 5px;
    background-color: var(--button-bg-color);
    color: var(--button-text-color);
    font-size: 16px;
    cursor: pointer;
    transition: background-color 0.3s;
}

button:hover {
    opacity: 0.9;
}

#total-expense {
    margin-top: 30px;
    padding: 20px;
    background-color: var(--result-bg-color);
    border-radius: 5px;
    text-align: left;
    font-size: 16px;
}

#total-expense h3 {
    text-align: center;
    margin-top: 0;
    font-size: 20px;
}

#total-expense p {
    margin: 10px 0;
    display: flex;
    justify-content: space-between;
}

#total-expense .final-total {
    font-weight: bold;
    font-size: 20px;
    margin-top: 15px;
    border-top: 2px solid var(--text-color);
    padding-top: 10px;
}

.theme-switcher {
    position: fixed;
    top: 20px;
    right: 20px;
    cursor: pointer;
}

.theme-switcher i {
    font-size: 24px;
}

#split-cost {
    margin-top: 30px;
    padding: 20px;
    background-color: var(--result-bg-color);
    border-radius: 5px;
}

.hidden {
    display: none;
}

#split-cost h3 {
    text-align: center;
    margin-top: 0;
}

#split-cost button {
    margin-top: 10px;
}

#split-result {
    text-align: center;
    margin-top: 20px;
    font-size: 18px;
}

#split-result strong {
    color: var(--button-bg-color);
}


@media (max-width: 600px) {
    .container {
        margin: 20px;
        padding: 20px;
    }
}