* {
    box-sizing: border-box;
}

body {
    margin: 0;
    font-family: Arial, sans-serif;
    background: #f2f7ef;
    color: #193b2a;
    text-align: center;
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 15px 8%;
    background: #194d35;
    color: white;
}

#theme {
    padding: 10px;
    cursor: pointer;
    border: none;
    border-radius: 8px;
}

.hero {
    padding: 75px 20px;
    background: linear-gradient(135deg, #dcebd6, #a8cda5);
}

h1 {
    font-size: 42px;
}

.hero span {
    color: #247447;
}

p {
    line-height: 1.7;
}

.btn {
    display: inline-block;
    padding: 13px 22px;
    margin: 10px;
    border: none;
    border-radius: 8px;
    background: #247447;
    color: white;
    text-decoration: none;
    cursor: pointer;
}

#planner {
    max-width: 650px;
    margin: 40px auto;
    padding: 30px 20px;
    background: white;
    border-radius: 15px;
}

input, select {
    display: block;
    width: 90%;
    margin: 15px auto;
    padding: 13px;
    border: 1px solid #ccd8ca;
    border-radius: 7px;
}

#result {
    margin-top: 20px;
    padding: 15px;
    line-height: 1.8;
}

.about {
    padding: 40px 20px;
    background: #dcebd6;
}

body.dark {
    background: #14251b;
    color: #f0f5ef;
}

body.dark #planner {
    background: #23392b;
}

body.dark .hero,
body.dark .about {
    background: #1c3526;
}

@media (max-width: 600px) {
    h1 {
        font-size: 32px;
    }

    #planner {
        margin: 20px 12px;
    }
}
```