<div class="wrapper">
  <div class="alert">
    <p>Your browser doesn't support CSS :has. Please view it in Safari 15.4 or the latest Chrome Canary.</p>
  </div>
</div>

<div style="margin-bottom: 1.5rem;">
  <label for="">Select theme</label>
  <select name="" id="themeSwitcher">
    <option value="default">Default</option>
    <option value="blueish">blueish</option>
  </select>
</div>

<header>
  <a class="logo" href="#">Logo</a>
  <ul>
    <li><a href="#">Home</a></li>
    <li><a href="#">About</a></li>
    <li><a href="#">Profile</a></li>
  </ul>
</header>

<div class="prose">
  <h1 style="font-size: 2rem; font-weight: bold;">This is a basic title just for the sake of testing</h1>
  <p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Sit ducimus cu!</p>

  <h2>Sub title in there</h2>
  <p>CSS :has is cool, isn't it?</p>
</div>

:root {
  --color-1: #74559c;
  --color-2: #f9f6fe;
}

html:has(option[value="default"]:checked) {
  --color-1: #74559c;
  --color-2: #f9f6fe;
}

html:has(option[value="blueish"]:checked) {
  --color-1: #466ec0;
  --color-2: #ebf0f7;
}

h1 {
  color: var(--color-1);
  margin-bottom: 2rem;
}

body {
  border-top: solid 10px var(--color-1);
  background-color: var(--color-2);
  min-height: 100vh;
}

.prose {
  > * + * {
    margin-top: 1rem;
  }

  h2 {
    font-weight: bold;
    font-size: 1.5rem;
    color: var(--color-1);
  }
}

header {
  display: flex;
  justify-content: space-between;
  background-color: var(--color-1);
  padding: 1rem;
  margin-bottom: 2rem;
  margin-left: -1rem;
  margin-right: -1rem;

  a {
    color: #fff;
    text-decoration: none;
  }

  .logo {
    font-weight: bold;
    font-size: 1rem;
  }

  ul {
    display: flex;
    gap: 1rem;
  }
}

/* Other styles */
.alert {
  display: block;
  padding: 1rem;
  text-align: center;
  margin-bottom: 1rem;
  background-color: #ffeeee;
  border: 1px solid red;
  border-radius: 5px;
  font-size: 0.75rem;
}

@supports selector(:has(*)) {
  .alert {
    display: none;
  }
}

body {
  font-family: "system-ui";
  padding: 1rem;
}

.eg {
  > * + * {
    margin-top: 2rem;
  }
}

.wrapper {
  max-width: 700px;
  margin: 2rem auto;
}

img {
  max-width: 100%;
}

*,
*:before,
*:after {
  box-sizing: border-box;
}