:root {
    /* Light theme variables */
    --bg-color: #f5f7fa;
    --container-bg: #fff;
    --text-color: #333;
    --heading-color: #2c3e50;
    --input-border: #ddd;
    --input-bg: #fff;
    --button-bg: #3498db;
    --button-hover: #2980b9;
    --button-text: #fff;
    --result-bg: #f8f9fa;
    --border-color: #eee;
    --instruction-color: #666;
}

.dark-theme {
    /* Dark theme variables */
    --bg-color: #121212;
    --container-bg: #1e1e1e;
    --text-color: #e0e0e0;
    --heading-color: #4da6ff;
    --input-border: #444;
    --input-bg: #2d2d2d;
    --button-bg: #0066cc;
    --button-hover: #004c99;
    --button-text: #fff;
    --result-bg: #2d2d2d;
    --border-color: #444;
    --instruction-color: #aaa;
}

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

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    line-height: 1.6;
    color: var(--text-color);
    background-color: var(--bg-color);
    padding: 20px;
    transition: background-color 0.3s, color 0.3s;
}

.container {
    max-width: 1000px;
    margin: 0 auto;
    background-color: var(--container-bg);
    border-radius: 10px;
    box-shadow: 0 2px 10px rgba(0, 0, 0, 0.2);
    padding: 20px;
}

header {
    text-align: center;
    margin-bottom: 20px;
    padding-bottom: 15px;
    border-bottom: 1px solid var(--border-color);
}

h1 {
    color: var(--heading-color);
    font-size: 1.8rem;
}

h2 {
    color: var(--heading-color);
    font-size: 1.4rem;
    margin-bottom: 15px;
}

.form-group {
    margin-bottom: 15px;
}

label {
    display: block;
    margin-bottom: 5px;
    font-weight: 600;
    color: var(--text-color);
}

.input-with-clear {
    position: relative;
    display: flex;
}

input, select {
    width: 100%;
    padding: 10px;
    border: 1px solid var(--input-border);
    border-radius: 5px;
    font-size: 16px;
    background-color: var(--input-bg);
    color: var(--text-color);
}

input:focus, select:focus {
    outline: none;
    border-color: var(--button-bg);
    box-shadow: 0 0 0 2px rgba(52, 152, 219, 0.2);
}

.clear-btn {
    position: relative;
    width: 38px;
    height: auto;
    border: 1px solid var(--input-border);
    border-left: none;
    border-radius: 0 5px 5px 0;
    background-color: var(--input-bg);
    color: var(--text-color);
    font-size: 20px;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 0;
    margin: 0;
}

.clear-btn:hover {
    background-color: var(--button-hover);
    color: var(--button-text);
}

.input-with-clear input {
    border-right: none;
    border-radius: 5px 0 0 5px;
}

.instruction-text {
    color: var(--instruction-color);
    font-style: italic;
    text-align: center;
    margin: 5px 0;
}

button {
    background-color: var(--button-bg);
    color: var(--button-text);
    border: none;
    padding: 12px 20px;
    border-radius: 5px;
    cursor: pointer;
    font-size: 16px;
    width: 100%;
    transition: background-color 0.2s;
}

button:hover {
    background-color: var(--button-hover);
}

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

.result-item {
    display: flex;
    justify-content: space-between;
    margin-bottom: 10px;
    padding-bottom: 10px;
    border-bottom: 1px solid var(--border-color);
}

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

/* Theme toggle button */
.theme-toggle {
    position: fixed;
    top: 20px;
    right: 20px;
    width: 40px;
    height: 40px;
    border-radius: 50%;
    background-color: var(--container-bg);
    color: var(--text-color);
    font-size: 20px;
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    border: 1px solid var(--input-border);
    box-shadow: 0 2px 5px rgba(0, 0, 0, 0.2);
    z-index: 1000;
    padding: 0;
}

.calculation-mode {
    margin-top: 30px;
    padding: 15px;
    background-color: var(--result-bg);
    border-radius: 5px;
    border-top: 2px solid var(--heading-color);
}

.calculation-mode label {
    display: block;
    margin-bottom: 8px;
    font-weight: 600;
}

.calculation-mode select {
    width: 100%;
    margin-bottom: 10px;
}

.calculation-note {
    font-size: 0.85rem;
    font-style: italic;
    color: var(--instruction-color);
    margin-top: 8px;
}

.signature {
    width: 100%;
    margin-top: 20px;
    text-align: center;
    font-size: 12px;
    font-family: sans-serif;
    padding: 5px 0;
    background-color: var(--bg-color);
    color: var(--instruction-color);
    opacity: 0.7;
    border-top: 1px dotted var(--border-color);
}

/* Media Queries for larger screens */
@media (min-width: 768px) {
    .container {
        padding: 30px;
    }
    
    h1 {
        font-size: 2.2rem;
    }
    
    .calculator-form {
        display: grid;
        grid-template-columns: 1fr 1fr;
        gap: 20px;
    }
    
    .form-group:last-of-type, 
    button#calculateBtn {
        grid-column: span 2;
    }
}